Add fail regex to detect supported warning flags correctly.

On Apple's gcc, the compiler emits a warning such as "warning: command
line option "-Wno-deprecated" is valid for C++/ObjC++ but not for C"
when it is passed that flag on its command line.

When testing for that flag with a plain C file, we should report that
the flag is unsupported for C files. The new FAIL_REGEX option added
here produces this behavior.

Modules/CheckCCompilerFlag.cmake

@@ -27,6 +27,8 @@ MACRO (CHECK_C_COMPILER_FLAG _FLAG _RESULT)
    SET(CMAKE_REQUIRED_DEFINITIONS "${_FLAG}")
    CHECK_C_SOURCE_COMPILES("int main(void) { return 0; }" ${_RESULT}
      # Some compilers do not fail with a bad flag
+     FAIL_REGEX "warning: command line option .* is valid for .* but not for C"
+                                                            # Apple gcc
      FAIL_REGEX "unrecognized .*option"                     # GNU
      FAIL_REGEX "ignoring unknown option"                   # MSVC
      FAIL_REGEX "warning D9002"                             # MSVC, any lang
@@ -36,4 +38,3 @@ MACRO (CHECK_C_COMPILER_FLAG _FLAG _RESULT)
      )
    SET (CMAKE_REQUIRED_DEFINITIONS "${SAFE_CMAKE_REQUIRED_DEFINITIONS}")
 ENDMACRO (CHECK_C_COMPILER_FLAG)
-