build: add -Wshorten-64-to-32 for non-hbd builds

C sources only for now

BUG=aomedia:445

Change-Id: I79ff81c42778e2836028240df70edf973555018c

build/cmake/aom_configure.cmake

@@ -133,6 +133,9 @@ else ()
   add_compiler_flag_if_supported("-Wuninitialized")
   add_compiler_flag_if_supported("-Wunused")
   add_compiler_flag_if_supported("-Wvla")
+  if (NOT CONFIG_HIGHBITDEPTH)
+    add_c_flag_if_supported("-Wshorten-64-to-32")
+  endif ()
 
   # Add -Wshadow only for C files to avoid massive gtest warning spam.
   add_c_flag_if_supported("-Wshadow")

configure

@@ -718,6 +718,11 @@ process_toolchain() {
         fi
         # gtest makes heavy use of undefined pre-processor symbols
         check_cflags "-Wundef" && add_cflags_only "-Wundef"
+        if ! enabled highbitdepth; then
+          # Avoid this warning for third_party C++ sources. Some reorganization
+          # would be needed to apply this only to test/*.cc.
+          check_cflags -Wshorten-64-to-32 && add_cflags_only -Wshorten-64-to-32
+        fi
     fi
 
     if enabled icc; then