diff --git a/build/cmake/toolchains/arm64-linux-gcc.cmake b/build/cmake/toolchains/arm64-linux-gcc.cmake
index fc4b277..64e460b 100644
--- a/build/cmake/toolchains/arm64-linux-gcc.cmake
+++ b/build/cmake/toolchains/arm64-linux-gcc.cmake
@@ -21,9 +21,15 @@
   set(CROSS aarch64-linux-gnu-)
 endif()
 
-set(CMAKE_C_COMPILER ${CROSS}gcc)
-set(CMAKE_CXX_COMPILER ${CROSS}g++)
-set(AS_EXECUTABLE ${CROSS}as)
+if(NOT CMAKE_C_COMPILER)
+  set(CMAKE_C_COMPILER ${CROSS}gcc)
+endif()
+if(NOT CMAKE_CXX_COMPILER)
+  set(CMAKE_CXX_COMPILER ${CROSS}g++)
+endif()
+if(NOT AS_EXECUTABLE)
+  set(AS_EXECUTABLE ${CROSS}as)
+endif()
 set(CMAKE_C_FLAGS_INIT "-march=armv8-a")
 set(CMAKE_CXX_FLAGS_INIT "-march=armv8-a")
 set(AOM_AS_FLAGS "-march=armv8-a")
diff --git a/build/cmake/toolchains/arm64-mingw-gcc.cmake b/build/cmake/toolchains/arm64-mingw-gcc.cmake
index a8e15cb..5472ed4 100644
--- a/build/cmake/toolchains/arm64-mingw-gcc.cmake
+++ b/build/cmake/toolchains/arm64-mingw-gcc.cmake
@@ -20,10 +20,18 @@
   set(CROSS aarch64-w64-mingw32-)
 endif()
 
-set(CMAKE_C_COMPILER ${CROSS}gcc)
-set(CMAKE_CXX_COMPILER ${CROSS}g++)
-set(CMAKE_AR ${CROSS}ar CACHE FILEPATH Archiver)
-set(CMAKE_RANLIB ${CROSS}ranlib CACHE FILEPATH Indexer)
+if(NOT CMAKE_C_COMPILER)
+  set(CMAKE_C_COMPILER ${CROSS}gcc)
+endif()
+if(NOT CMAKE_CXX_COMPILER)
+  set(CMAKE_CXX_COMPILER ${CROSS}g++)
+endif()
+if(NOT CMAKE_AR)
+  set(CMAKE_AR ${CROSS}ar CACHE FILEPATH Archiver)
+endif()
+if(NOT CMAKE_RANLIB)
+  set(CMAKE_RANLIB ${CROSS}ranlib CACHE FILEPATH Indexer)
+endif()
 
 # No runtime cpu detect for arm64-mingw-gcc.
 set(CONFIG_RUNTIME_CPU_DETECT 0 CACHE STRING "")
diff --git a/build/cmake/toolchains/armv7-linux-gcc.cmake b/build/cmake/toolchains/armv7-linux-gcc.cmake
index 26c028f..1201538 100644
--- a/build/cmake/toolchains/armv7-linux-gcc.cmake
+++ b/build/cmake/toolchains/armv7-linux-gcc.cmake
@@ -25,9 +25,15 @@
   set(AOM_EXTRA_TOOLCHAIN_FLAGS "-mfloat-abi=softfp")
 endif()
 
-set(CMAKE_C_COMPILER ${CROSS}gcc)
-set(CMAKE_CXX_COMPILER ${CROSS}g++)
-set(AS_EXECUTABLE ${CROSS}as)
+if(NOT CMAKE_C_COMPILER)
+  set(CMAKE_C_COMPILER ${CROSS}gcc)
+endif()
+if(NOT CMAKE_CXX_COMPILER)
+  set(CMAKE_CXX_COMPILER ${CROSS}g++)
+endif()
+if(NOT AS_EXECUTABLE)
+  set(AS_EXECUTABLE ${CROSS}as)
+endif()
 set(CMAKE_C_FLAGS_INIT "-march=armv7-a -mfpu=vfpv3 \
                           ${AOM_EXTRA_TOOLCHAIN_FLAGS}")
 set(CMAKE_CXX_FLAGS_INIT "-march=armv7-a -mfpu=vfpv3 \
diff --git a/build/cmake/toolchains/armv7-mingw-gcc.cmake b/build/cmake/toolchains/armv7-mingw-gcc.cmake
index 2dc4b18..8a92891 100644
--- a/build/cmake/toolchains/armv7-mingw-gcc.cmake
+++ b/build/cmake/toolchains/armv7-mingw-gcc.cmake
@@ -20,10 +20,18 @@
   set(CROSS armv7-w64-mingw32-)
 endif()
 
-set(CMAKE_C_COMPILER ${CROSS}gcc)
-set(CMAKE_CXX_COMPILER ${CROSS}g++)
-set(CMAKE_AR ${CROSS}ar CACHE FILEPATH Archiver)
-set(CMAKE_RANLIB ${CROSS}ranlib CACHE FILEPATH Indexer)
+if(NOT CMAKE_C_COMPILER)
+  set(CMAKE_C_COMPILER ${CROSS}gcc)
+endif()
+if(NOT CMAKE_CXX_COMPILER)
+  set(CMAKE_CXX_COMPILER ${CROSS}g++)
+endif()
+if(NOT CMAKE_AR)
+  set(CMAKE_AR ${CROSS}ar CACHE FILEPATH Archiver)
+endif()
+if(NOT CMAKE_RANLIB)
+  set(CMAKE_RANLIB ${CROSS}ranlib CACHE FILEPATH Indexer)
+endif()
 
 # No runtime cpu detect for armv7-mingw-gcc.
 set(CONFIG_RUNTIME_CPU_DETECT 0 CACHE STRING "")
diff --git a/build/cmake/toolchains/mips32-linux-gcc.cmake b/build/cmake/toolchains/mips32-linux-gcc.cmake
index ad5ebff..15bed17 100644
--- a/build/cmake/toolchains/mips32-linux-gcc.cmake
+++ b/build/cmake/toolchains/mips32-linux-gcc.cmake
@@ -62,9 +62,15 @@
   set(CMAKE_EXE_LINKER_FLAGS "-mfp64 ${CMAKE_EXE_LINKER_FLAGS}")
 endif()
 
-set(CMAKE_C_COMPILER ${CROSS}gcc)
-set(CMAKE_CXX_COMPILER ${CROSS}g++)
-set(AS_EXECUTABLE ${CROSS}as)
+if(NOT CMAKE_C_COMPILER)
+  set(CMAKE_C_COMPILER ${CROSS}gcc)
+endif()
+if(NOT CMAKE_CXX_COMPILER)
+  set(CMAKE_CXX_COMPILER ${CROSS}g++)
+endif()
+if(NOT AS_EXECUTABLE)
+  set(AS_EXECUTABLE ${CROSS}as)
+endif()
 set(CMAKE_C_FLAGS_INIT "-EL ${MIPS_CFLAGS}")
 set(CMAKE_CXX_FLAGS_INIT "-EL ${MIPS_CXXFLAGS}")
 set(CMAKE_EXE_LINKER_FLAGS_INIT "-EL ${MIPS_CXXFLAGS}")
diff --git a/build/cmake/toolchains/mips64-linux-gcc.cmake b/build/cmake/toolchains/mips64-linux-gcc.cmake
index 0af99245..93e7d22 100644
--- a/build/cmake/toolchains/mips64-linux-gcc.cmake
+++ b/build/cmake/toolchains/mips64-linux-gcc.cmake
@@ -39,9 +39,15 @@
       "-mips64r6 -mabi64 -mfp64 ${CMAKE_EXE_LINKER_FLAGS}")
 endif()
 
-set(CMAKE_C_COMPILER ${CROSS}gcc)
-set(CMAKE_CXX_COMPILER ${CROSS}g++)
-set(AS_EXECUTABLE ${CROSS}as)
+if(NOT CMAKE_C_COMPILER)
+  set(CMAKE_C_COMPILER ${CROSS}gcc)
+endif()
+if(NOT CMAKE_CXX_COMPILER)
+  set(CMAKE_CXX_COMPILER ${CROSS}g++)
+endif()
+if(NOT AS_EXECUTABLE)
+  set(AS_EXECUTABLE ${CROSS}as)
+endif()
 set(CMAKE_C_FLAGS_INIT "-EL ${MIPS_CFLAGS}")
 set(CMAKE_CXX_FLAGS_INIT "-EL ${MIPS_CXXFLAGS}")
 set(CMAKE_EXE_LINKER_FLAGS_INIT "-EL ${MIPS_CXXFLAGS}")
diff --git a/build/cmake/toolchains/ppc-linux-gcc.cmake b/build/cmake/toolchains/ppc-linux-gcc.cmake
index 54db99b..ab0efea 100644
--- a/build/cmake/toolchains/ppc-linux-gcc.cmake
+++ b/build/cmake/toolchains/ppc-linux-gcc.cmake
@@ -21,9 +21,15 @@
   set(CROSS powerpc64le-unknown-linux-gnu-)
 endif()
 
-set(CMAKE_C_COMPILER ${CROSS}gcc)
-set(CMAKE_CXX_COMPILER ${CROSS}g++)
-set(AS_EXECUTABLE ${CROSS}as)
+if(NOT CMAKE_C_COMPILER)
+  set(CMAKE_C_COMPILER ${CROSS}gcc)
+endif()
+if(NOT CMAKE_CXX_COMPILER)
+  set(CMAKE_CXX_COMPILER ${CROSS}g++)
+endif()
+if(NOT AS_EXECUTABLE)
+  set(AS_EXECUTABLE ${CROSS}as)
+endif()
 set(CMAKE_SYSTEM_PROCESSOR "ppc")
 
 set(CONFIG_RUNTIME_CPU_DETECT 0 CACHE STRING "")
diff --git a/build/cmake/toolchains/x86-mingw-gcc.cmake b/build/cmake/toolchains/x86-mingw-gcc.cmake
index 2e9a9a8..f75728f 100644
--- a/build/cmake/toolchains/x86-mingw-gcc.cmake
+++ b/build/cmake/toolchains/x86-mingw-gcc.cmake
@@ -23,7 +23,15 @@
   set(CROSS i686-w64-mingw32-)
 endif()
 
-set(CMAKE_C_COMPILER ${CROSS}gcc)
-set(CMAKE_CXX_COMPILER ${CROSS}g++)
-set(CMAKE_AR ${CROSS}ar CACHE FILEPATH Archiver)
-set(CMAKE_RANLIB ${CROSS}ranlib CACHE FILEPATH Indexer)
+if(NOT CMAKE_C_COMPILER)
+  set(CMAKE_C_COMPILER ${CROSS}gcc)
+endif()
+if(NOT CMAKE_CXX_COMPILER)
+  set(CMAKE_CXX_COMPILER ${CROSS}g++)
+endif()
+if(NOT CMAKE_AR)
+  set(CMAKE_AR ${CROSS}ar CACHE FILEPATH Archiver)
+endif()
+if(NOT CMAKE_RANLIB)
+  set(CMAKE_RANLIB ${CROSS}ranlib CACHE FILEPATH Indexer)
+endif()
diff --git a/build/cmake/toolchains/x86_64-mingw-gcc.cmake b/build/cmake/toolchains/x86_64-mingw-gcc.cmake
index 4b2d28d..56e9b6e 100644
--- a/build/cmake/toolchains/x86_64-mingw-gcc.cmake
+++ b/build/cmake/toolchains/x86_64-mingw-gcc.cmake
@@ -20,7 +20,15 @@
   set(CROSS x86_64-w64-mingw32-)
 endif()
 
-set(CMAKE_C_COMPILER ${CROSS}gcc)
-set(CMAKE_CXX_COMPILER ${CROSS}g++)
-set(CMAKE_AR ${CROSS}ar CACHE FILEPATH Archiver)
-set(CMAKE_RANLIB ${CROSS}ranlib CACHE FILEPATH Indexer)
+if(NOT CMAKE_C_COMPILER)
+  set(CMAKE_C_COMPILER ${CROSS}gcc)
+endif()
+if(NOT CMAKE_CXX_COMPILER)
+  set(CMAKE_CXX_COMPILER ${CROSS}g++)
+endif()
+if(NOT CMAKE_AR)
+  set(CMAKE_AR ${CROSS}ar CACHE FILEPATH Archiver)
+endif()
+if(NOT CMAKE_RANLIB)
+  set(CMAKE_RANLIB ${CROSS}ranlib CACHE FILEPATH Indexer)
+endif()