cmake: treat warnings as error in compiler flag tests

CMakeLists.txt

@@ -106,16 +106,18 @@ function (die msg)
 endfunction ()
 
 function (add_c_flag_if_supported flag var)
-  string(REPLACE "-" "_" supported ${flag}_c)
-  check_c_compiler_flag(${flag} ${supported})
+  set(TMP "-Werror ${flag}")
+  string(REGEX REPLACE "[- ]" "_" supported ${TMP}_c)
+  check_c_compiler_flag(${TMP} ${supported})
   if(${${supported}})
     set(${var} "${${var}} ${flag}" PARENT_SCOPE)
   endif()
 endfunction()
 
 function (add_cxx_flag_if_supported flag var)
-  string(REPLACE "-" "_" supported ${flag}_cxx)
-  check_cxx_compiler_flag(${flag} ${supported})
+  set(TMP "-Werror ${flag}")
+  string(REGEX REPLACE "[- ]" "_" supported ${TMP}_cxx)
+  check_cxx_compiler_flag(${TMP} ${supported})
   if(${${supported}})
     set(${var} "${${var}} ${flag}" PARENT_SCOPE)
   endif()