Clang-Tidy: Allow use with GCC but warn about possible failure

The compilation using GCC + Clang-Tidy succeeded for me and Sybren
(with some linker caveats for Sybren) but seems that it is doable
to make GCC + Clang-Tidy to officially work for Blender.

Now it should be possible to enable Clang-Tidy doing something like

  make developer debug BUILD_CMAKE_ARGS='-DWITH_CLANG_TIDY=ON'

source/CMakeLists.txt

@@ -24,7 +24,7 @@ endif()
 
 if(WITH_CLANG_TIDY)
   if(NOT CMAKE_C_COMPILER_ID MATCHES "Clang")
-    message(FATAL_ERROR "Currently Clang-Tidy is only supported when using Clang compiler")
+    message(WARNING "Currently Clang-Tidy might fail with GCC toolchain, switch to Clang toolchain if that happens")
   endif()
 
   find_package(ClangTidy REQUIRED)