Merge pull request #13432 from bloerwald/fix_compiler_detection_on_unix

cmake: platform: also detect clang by C_COMPILER_ID: C_COMPILER may vary
(cherry picked from commit 774e22ec4a8859664ace0a532c2ab4ae220e5cc9)
Merge branch '3.3.5' of https://github.com/TrinityCore/TrinityCore into ghbb