The solution arrived at in commit e74dd00f5 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 68fc227dd,
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
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