cmake: Add -Wundef for C sources

It was added to configure in ce32f705a878f5dffc0a64998a02893366700260

Change-Id: I67a4155861d5c012dc18702b3d4b9abe8e0e342b

build/cmake/aom_configure.cmake

diff --git a/build/cmake/aom_configure.cmake b/build/cmake/aom_configure.cmake
index 78426f2..3c9402d 100644
--- a/build/cmake/aom_configure.cmake
+++ b/build/cmake/aom_configure.cmake
@@ -143,6 +143,9 @@
   # Add -Wshadow only for C files to avoid massive gtest warning spam.
   add_c_flag_if_supported("-Wshadow")
 
+  # Add -Wundef only for C files to avoid massive gtest warning spam.
+  add_c_flag_if_supported("-Wundef")
+
   if (ENABLE_WERROR)
     add_compiler_flag_if_supported("-Werror")
   endif ()