Yet further rethinking of build changes for macOS Mojave.

The solution arrived at in commit e74dd00f presumes that the compiler
has a suitable default -isysroot setting ... but further experience
shows that in many combinations of macOS version, XCode version, Xcode
command line tools version, and phase of the moon, Apple's compiler
will *not* supply a default -isysroot value.

We could potentially go back to the approach used in commit 68fc227d,
but I don't have a lot of faith in the reliability or life expectancy of
that either.  Let's just revert to the approach already shipped in 11.0,
namely specifying an -isysroot switch globally.  As a partial response to
the concerns raised by Jakob Egger, adjust the contents of Makefile.global
to look like

CPPFLAGS = -isysroot $(PG_SYSROOT) ...
PG_SYSROOT = /path/to/sysroot

This allows overriding the sysroot path at build time in a relatively
painless way.

Add documentation to installation.sgml about how to use the PG_SYSROOT
option.  I also took the opportunity to document how to work around
macOS's "System Integrity Protection" feature.

As before, back-patch to all supported versions.

Discussion: https://postgr.es/m/20840.1537850987@sss.pgh.pa.us