WCDH: Fix the C_STANDARD property in the tests.

The CXX_STANDARD property validates the values allowed for it,
and '90' is not a valid value for it.  However, the validation is only
done for languages enabled for the target, and as C_undefined has no
CXX files, that language is not enabled for it.

This amends commit v3.1.0-rc3~23^2 (WCDH: Make the header -Wundef
safe for the C language., 2014-11-24).

Tests/Module/WriteCompilerDetectionHeader/CMakeLists.txt

@@ -71,7 +71,7 @@ if (CMAKE_C_COMPILER_ID STREQUAL "GNU"
     OR CMAKE_C_COMPILER_ID STREQUAL "Clang"
     OR CMAKE_C_COMPILER_ID STREQUAL "AppleClang")
   add_executable(C_undefined c_undefined.c)
-  set_property(TARGET C_undefined PROPERTY CXX_STANDARD 90)
+  set_property(TARGET C_undefined PROPERTY C_STANDARD 90)
   target_compile_options(C_undefined PRIVATE -Werror=undef)
 endif()