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  `__clang__`, in some cases this has been replaced by explicitly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

intern/atomic/intern/atomic_ops_utils.h

@@ -106,8 +106,7 @@
 /* Copied from BLI_utils... */
 /* C++ can't use _Static_assert, expects static_assert() but c++0x only,
  * Coverity also errors out. */
-#if (!defined(__cplusplus)) && (!defined(__COVERITY__)) && \
-    (defined(__GNUC__) && ((__GNUC__ * 100 + __GNUC_MINOR__) >= 406)) /* gcc4.6+ only */
+#if (!defined(__cplusplus)) && (!defined(__COVERITY__)) && (defined(__GNUC__)) /* GCC only. */
 #  define ATOMIC_STATIC_ASSERT(a, msg) __extension__ _Static_assert(a, msg);
 #else
 /* Code adapted from http://www.pixelbeat.org/programming/gcc/static_assert.html */

intern/atomic/tests/atomic_test.cc

@@ -6,12 +6,8 @@
 #include "testing/testing.h"
 
 #ifdef __GNUC__
-#  if (__GNUC__ * 100 + __GNUC_MINOR__) >= 406 /* gcc4.6+ only */
-#    pragma GCC diagnostic error "-Wsign-compare"
-#  endif
-#  if (__GNUC__ * 100 + __GNUC_MINOR__) >= 408
-#    pragma GCC diagnostic error "-Wsign-conversion"
-#  endif
+#  pragma GCC diagnostic error "-Wsign-compare"
+#  pragma GCC diagnostic error "-Wsign-conversion"
 #endif
 
 /* -------------------------------------------------------------------- */