Consolidate makefile code for setting top_srcdir, srcdir and VPATH.

Responsibility was formerly split between Makefile.global and pgxs.mk.
As a result of commit b58233c7, in the
PGXS case, these variables were unset while parsing Makefile.global and
callees.  Inclusion of Makefile.custom did not work from PGXS, and the
subtle difference seemed like a recipe for future bugs.  Back-patch to
9.4, where that commit first appeared.