Add -Wimplicit-function-declaration for C only.

Fixes the following warning:
"cc1plus: warning: command-line option '-Wimplicit-function-declaration'
is valid for C/ObjC but not for C++"

BUG=aomedia:2886

Change-Id: Iaf8ef0dfad958be0ae12f22f9771609168199362

build/cmake/aom_configure.cmake

diff --git a/build/cmake/aom_configure.cmake b/build/cmake/aom_configure.cmake
index 150552d..a19dc33 100644
--- a/build/cmake/aom_configure.cmake
+++ b/build/cmake/aom_configure.cmake
@@ -266,7 +266,6 @@
   add_compiler_flag_if_supported("-Wdisabled-optimization")
   add_compiler_flag_if_supported("-Wextra")
   add_compiler_flag_if_supported("-Wfloat-conversion")
-  add_compiler_flag_if_supported("-Wimplicit-function-declaration")
   add_compiler_flag_if_supported("-Wlogical-op")
   add_compiler_flag_if_supported("-Wpointer-arith")
   add_compiler_flag_if_supported("-Wshorten-64-to-32")
@@ -292,6 +291,9 @@
     add_cxx_flag_if_supported("-Wstack-usage=240000")
   endif()
 
+  # Flags valid for C, but not for C++.
+  add_c_flag_if_supported("-Wimplicit-function-declaration")
+
   # Add -Wshadow only for C files to avoid massive gtest warning spam.
   add_c_flag_if_supported("-Wshadow")