pgsql: 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

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/175f716a8f37c0325cc3f683dad242536f9515d0

Modified Files
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src/template/darwin | 19 +++++++++++++------
1 file changed, 13 insertions(+), 6 deletions(-)