Assorted build improvements:

• Add a build conflict for commonmark-cmark-* when DOCS are enabled. This prevents a failure later on in the build. [0]
• Add a new option BE_AMDGPU which can be used to enable the AMDGPU backed used by mesa when BE_NATIVE or BE_FREEBSD is set. Enable this option by default to limit later surprises. [1]
• New option PYCLANG to add python binding for clang. [2]

PR: 239636 [0], 230789 [1], 239990 [2]
Submitted by: chardon.frederic@gmail.com [2]
Sponsored by: DARPA, AFRL