Further tweaking of PG_SYSROOT heuristics for macOS.

Further tweaking of PG_SYSROOT heuristics for macOS.

It emerges that in some phases of the moon (perhaps to do with
directory entry order?), xcrun will report that the SDK path is
  /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk
which is normally a symlink to a version-numbered sibling directory.
Our heuristic to skip non-version-numbered pathnames was rejecting
that, which is the wrong thing to do.  We'd still like to end up
with a version-numbered PG_SYSROOT value, but we can have that by
dereferencing the symlink.

Like the previous fix, back-patch to all supported versions.

Discussion: https://postgr.es/m/522433.1611089678@sss.pgh.pa.us