Re: PG vs macOS Mojave

So it seems like there are two ways we could go about this. One is
to go back to the scheme of adding an -isysroot switch to CPPFLAGS,
where it'd have global effects. We could make this slightly less
painful for scenarios like Jakob's if we set things up in Makefile.global
this way:

CPPFLAGS = -isysroot $(PG_SYSROOT)
PG_SYSROOT = /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk

and then, if you need to build on a different SDK version without
reconfiguring, you can do something like "make PG_SYSROOT=/proper/path".
I coded this up, as attached, and it seems to work but it's still not all
that friendly for such cases.

The other idea that's occurred to me is to go back to the scheme of
commit 68fc227dd, where we inject the sysroot path into just the -I
switches used for PL/Perl and PL/Tcl. We could improve on that
commit by injecting it symbolically similar to what I did here, ie
what ends up in the configure output is

PG_SYSROOT = /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk

perl_includespec = -I $(PG_SYSROOT)/System/Library/Perl/5.18/darwin-thread-multi-2level/CORE

Then somebody who wants to build on a different SDK version still needs
to do "make PG_SYSROOT=/proper/path", but only if they're trying to
build PL/Perl or related extensions. So this second way seems uglier
in some sense but less likely to cause problems for most people.

Either way I guess we'd need to document it rather than just hoping
it's invisible.

Thoughts?

regards, tom lane