Fix CHECK_(C|CXX)_COMPILER_FLAG macro test

The flag "-_this_is_not_a_flag_" was not rejected by GCC 4.0 on older
Mac OS X.  We now use "---_this_is_not_a_flag_" instead, which will
hopefully be rejected by all compilers.

Tests/TryCompile/CMakeLists.txt

@@ -202,10 +202,10 @@ TEST_ASSERT(CXX_RUN_SHOULD_WORK "CHECK_CXX_SOURCE_RUNS() failed")
 
 UNSET(C_BOGUS_FLAG CACHE)
 INCLUDE(CheckCCompilerFlag)
-CHECK_C_COMPILER_FLAG(-_this_is_not_a_flag_ C_BOGUS_FLAG)
+CHECK_C_COMPILER_FLAG(---_this_is_not_a_flag_ C_BOGUS_FLAG)
 TEST_FAIL(C_BOGUS_FLAG "CHECK_C_COMPILER_FLAG() succeeded, but should have failed")
 
 UNSET(CXX_BOGUS_FLAG CACHE)
 INCLUDE(CheckCXXCompilerFlag)
-CHECK_CXX_COMPILER_FLAG(-_this_is_not_a_flag_ CXX_BOGUS_FLAG)
+CHECK_CXX_COMPILER_FLAG(---_this_is_not_a_flag_ CXX_BOGUS_FLAG)
 TEST_FAIL(CXX_BOGUS_FLAG "CHECK_CXX_COMPILER_FLAG() succeeded, but should have failed")