Cleanup: remove workarounds and version checks for unsupported compilers

Match minimum supported versions from the WIKI [0] by raising them to:

- GCC 9.3.1
- CLANG 8.0
- MVCS 2019 (16.9.16 / 1928)

Details:

- Add CMake checks that ensure supported compiler versions early on.
- Previously GCC per-processor version checks served to exclude
  `__clang__`, in some cases this has been replaced by explicitly
  excluding `__clang__`. This was needed as CLANG treated some of these
  flags differently to GCC, causing the build to fail.
- Remove USE_APPLE_OMP_FIX GCC-4.2 OpenMP workaround.
- Remove linking error workaround for old MSVC versions.

[0]: https://wiki.blender.org/wiki/Building_Blender

Reviewed by: brecht, LazyDodo

Ref D16068

extern/gflags/CMakeLists.txt

@@ -3,9 +3,7 @@
 
 # Too noisy for code we don't maintain.
 if(CMAKE_COMPILER_IS_GNUCC)
-  if(NOT "${CMAKE_CXX_COMPILER_VERSION}" VERSION_LESS "8.0")
-    add_cxx_flag("-Wno-cast-function-type")
-  endif()
+  add_cxx_flag("-Wno-cast-function-type")
 endif()
 
 set(INC