[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/applications/network/telnet/CMakeLists.txt

@@ -24,10 +24,15 @@ list(APPEND SOURCE
 add_executable(telnet ${SOURCE} telnet.rc)
 target_link_libraries(telnet cppstl)
 set_target_cpp_properties(telnet WITH_EXCEPTIONS)
+
 if (NOT MSVC)
-    target_compile_options(telnet PRIVATE -Wno-restrict $<$<COMPILE_LANGUAGE:CXX>:-Wno-narrowing>)
     target_compile_definitions(telnet PRIVATE _CRT_NONSTDC_NO_DEPRECATE)
 endif()
+
+if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
+    target_compile_options(telnet PRIVATE -Wno-restrict $<$<COMPILE_LANGUAGE:CXX>:-Wno-narrowing>)
+endif()
+
 set_module_type(telnet win32cui)
 add_importlibs(telnet ws2_32 user32 msvcrt kernel32 ntdll)
 add_pch(telnet precomp.h SOURCE)

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()