Make some fixes to allow building Postgres on macOS 10.14 ("Mojave").

Apple's latest rearrangements of the system-supplied headers have broken
building of PL/Perl and PL/Tcl.  The only practical way to fix PL/Tcl is to
start using the "-isysroot" compiler flag to point to SDK-supplied headers,
as Apple expects.  We must also start distinguishing where to find Perl's
headers from where to find its shared library; but that seems like good
cleanup anyway.

Extensions that formerly did something like -I$(perl_archlibexp)/CORE
should now do -I$(perl_includedir)/CORE instead.  perl_archlibexp
is still the place to look for libperl.so, though.

If for some reason you don't like the default -isysroot setting, you can
override that by setting PG_SYSROOT in configure's arguments.  I don't
currently think people would need to do so, unless maybe for cross-version
build purposes.

In addition, teach configure where to find tclConfig.sh.  Our traditional
method of searching $auto_path hasn't worked for the last couple of macOS
releases, and it now seems clear that Apple's not going to change that.
The workaround of manually specifying --with-tclconfig was annoying
already, but Mojave's made it a lot more so because the sysroot path now
has to be included as well.  Let's just wire the knowledge into configure
instead.  To avoid breaking builds against non-default Tcl installations
(e.g. MacPorts) wherein the $auto_path method probably still works,
arrange to try the additional case only after all else has failed.

Back-patch to all supported versions, since at least the buildfarm
cares about that.  The changes are set up to not do anything on macOS
releases that are old enough to not have functional sysroot trees.

configure.in

@@ -968,6 +968,15 @@ You might have to rebuild your Perl installation.  Refer to the
 documentation for details.  Use --without-perl to disable building
 PL/Perl.])
   fi
+  # On most platforms, archlibexp is also where the Perl include files live ...
+  perl_includedir="$perl_archlibexp"
+  # ... but on some macOS versions, we must look under $PG_SYSROOT instead
+  if test x"$PG_SYSROOT" != x"" ; then
+    if test -d "$PG_SYSROOT$perl_archlibexp" ; then
+      perl_includedir="$PG_SYSROOT$perl_archlibexp"
+    fi
+  fi
+  AC_SUBST(perl_includedir)dnl
   PGAC_CHECK_PERL_EMBED_CCFLAGS
   PGAC_CHECK_PERL_EMBED_LDFLAGS
 fi
@@ -2103,7 +2112,7 @@ fi
 # check for <perl.h>
 if test "$with_perl" = yes; then
   ac_save_CPPFLAGS=$CPPFLAGS
-  CPPFLAGS="$CPPFLAGS -I$perl_archlibexp/CORE"
+  CPPFLAGS="$CPPFLAGS -I$perl_includedir/CORE"
   AC_CHECK_HEADER(perl.h, [], [AC_MSG_ERROR([header file <perl.h> is required for Perl])],
                   [#include <EXTERN.h>])
   # While we're at it, check that we can link to libperl.