correctly detect unsupported compiler flags

in gcc `-Wno-unsupported-something` will not be an error or even a warning,
so cmake will think the flag is supported. But if there's any other
warning during compilation, for any reason, unknown option will
be a warning too. Or an error when -Werror, even if that "other warning"
would not be an error on itself.

So we need to detect whether `-Wno-unsupported-something` is *really*
supported. Luckily, `-Wunsupported-something` will always fail with an
error.

So, whenever there's a need to detect if -Wno-something is supported,
test -Wsomething instead.

cmake/check_compiler_flag.cmake

@@ -32,25 +32,25 @@ MACRO (MY_CHECK_CXX_COMPILER_FLAG flag)
   SET(CMAKE_REQUIRED_FLAGS "${SAVE_CMAKE_REQUIRED_FLAGS}")
 ENDMACRO()
 
-FUNCTION(MY_CHECK_AND_SET_COMPILER_FLAG flag)
+FUNCTION(MY_CHECK_AND_SET_COMPILER_FLAG flag_to_set)
   # At the moment this is gcc-only.
   # Let's avoid expensive compiler tests on Windows:
   IF(WIN32)
     RETURN()
   ENDIF()
-  MY_CHECK_C_COMPILER_FLAG(${flag})
-  MY_CHECK_CXX_COMPILER_FLAG(${flag})
-  STRING(REGEX REPLACE "[-,= +]" "_" result "${flag}")
+  STRING(REGEX REPLACE "^-Wno-" "-W" flag_to_check ${flag_to_set})
+  MY_CHECK_C_COMPILER_FLAG(${flag_to_check})
+  MY_CHECK_CXX_COMPILER_FLAG(${flag_to_check})
+  STRING(REGEX REPLACE "[-,= +]" "_" result "${flag_to_check}")
   FOREACH(lang C CXX)
     IF (HAVE_${lang}_${result})
       IF(ARGN)
         FOREACH(type ${ARGN})
-          SET(CMAKE_${lang}_FLAGS_${type} "${CMAKE_${lang}_FLAGS_${type}} ${flag}" PARENT_SCOPE)
+          SET(CMAKE_${lang}_FLAGS_${type} "${CMAKE_${lang}_FLAGS_${type}} ${flag_to_set}" PARENT_SCOPE)
         ENDFOREACH()
       ELSE()
-        SET(CMAKE_${lang}_FLAGS "${CMAKE_${lang}_FLAGS} ${flag}" PARENT_SCOPE)
+        SET(CMAKE_${lang}_FLAGS "${CMAKE_${lang}_FLAGS} ${flag_to_set}" PARENT_SCOPE)
       ENDIF()
     ENDIF()
   ENDFOREACH()
 ENDFUNCTION()
-