Merge branch 'dl/posix-unused-warning-clang'

The UNUSED macro in 'compat/posix.h' has been updated to use a
newly introduced GIT_CLANG_PREREQ macro for compiler version
checks, and the existing GIT_GNUC_PREREQ macro has been modernized
to use explicit major/minor comparisons rather than bit-shifting.

* dl/posix-unused-warning-clang:
  compat/posix.h: simplify GIT_GNUC_PREREQ() comparison
  compat/posix.h: clean up GIT_GNUC_PREREQ() and UNUSED
  compat/posix.h: enable UNUSED warning messages for Clang