[CMAKE] Some options are only supported by GCC, don't use them for clang

These are (so far):
-Wno-format-overflow
-Wno-nonnull-compare
-Wno-old-style-declaration
-Wno-unused-but-set-variable

base/services/tftpd/CMakeLists.txt

@@ -4,6 +4,6 @@ set_module_type(tftpd win32cui)
 add_importlibs(tftpd advapi32 ws2_32 iphlpapi msvcrt kernel32)
 add_cd_file(TARGET tftpd DESTINATION reactos/system32 FOR all)
 
-if(GCC OR CLANG)
+if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
     target_compile_options(tftpd PRIVATE -Wno-format-overflow)
 endif()