Do not use hardcoded install paths

BUG=aomedia:1522

Change-Id: If322249bd79218aed8c366b132c1bd78677e93ef
Signed-off-by: Robert-André Mauchin <zebob.m@gmail.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index a3b2046..d3b8364 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -54,6 +54,10 @@
 
 set(AOM_ROOT "${CMAKE_CURRENT_SOURCE_DIR}")
 set(AOM_CONFIG_DIR "${CMAKE_CURRENT_BINARY_DIR}")
+set(INCLUDE_INSTALL_DIR "${CMAKE_INSTALL_PREFIX}/include" CACHE PATH
+    "Installation path of includes")
+set(LIB_INSTALL_DIR "${CMAKE_INSTALL_PREFIX}/lib" CACHE PATH
+    "Installation path of libraries")
 
 if ("${AOM_ROOT}" STREQUAL "${AOM_CONFIG_DIR}")
   message(FATAL_ERROR
@@ -649,10 +653,10 @@
 set(AOM_INSTALL_LIBS aom)
 
 install(FILES ${AOM_INSTALL_INCS}
-        DESTINATION "${CMAKE_INSTALL_PREFIX}/include/aom")
+        DESTINATION "${INCLUDE_INSTALL_DIR}/aom")
 install(FILES "${AOM_CONFIG_DIR}/aom.pc"
-        DESTINATION "${CMAKE_INSTALL_PREFIX}/lib/pkgconfig")
-install(TARGETS ${AOM_INSTALL_LIBS} DESTINATION "${CMAKE_INSTALL_PREFIX}/lib")
+        DESTINATION "${LIB_INSTALL_DIR}/pkgconfig")
+install(TARGETS ${AOM_INSTALL_LIBS} DESTINATION "${LIB_INSTALL_DIR}")
 
 if (ENABLE_EXAMPLES)
   install(TARGETS ${AOM_INSTALL_BINS} DESTINATION "${CMAKE_INSTALL_PREFIX}/bin")