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

src/template/darwin

@@ -7,13 +7,20 @@
 if test x"$PG_SYSROOT" = x"" ; then
   # This is far more complicated than it ought to be.  We first ask
   # "xcrun --show-sdk-path", which seems to match the default -isysroot
-  # setting of Apple's compilers.  However, that may produce no result or
-  # a result that is not version-specific (i.e., just ".../SDKs/MacOSX.sdk").
-  # Using a version-specific sysroot seems desirable, so if there are not
-  # digits in the directory name, try "xcrun --sdk macosx --show-sdk-path";
-  # and if that still doesn't work, fall back to asking xcodebuild,
-  # which is often a good deal slower.
+  # setting of Apple's compilers.
   PG_SYSROOT=`xcrun --show-sdk-path 2>/dev/null`
+  # That may fail, or produce a result that is not version-specific (i.e.,
+  # just ".../SDKs/MacOSX.sdk").  Using a version-specific sysroot seems
+  # desirable, so if the path is a non-version-specific symlink, expand it.
+  if test -L "$PG_SYSROOT"; then
+    if expr x"$PG_SYSROOT" : '.*[0-9]\.[0-9][^/]*$' >/dev/null ; then : okay
+    else
+      PG_SYSROOT=`expr "$PG_SYSROOT" : '\(.*\)/'`/`readlink "$PG_SYSROOT"`
+    fi
+  fi
+  # If there are still not digits in the directory name, try
+  # "xcrun --sdk macosx --show-sdk-path"; and if that still doesn't work,
+  # fall back to asking xcodebuild, which is often a good deal slower.
   if expr x"$PG_SYSROOT" : '.*[0-9]\.[0-9][^/]*$' >/dev/null ; then : okay
   else
     PG_SYSROOT=`xcrun --sdk macosx --show-sdk-path 2>/dev/null`