pgsql: Consolidate makefile code for setting top_srcdir, srcdir and VPA

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 b58233c71b93a32fcab7219585cafc25a27eb769, 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.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/c7446194fa8fbbb9e8d948668bb47563ab58f45f

Modified Files
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src/Makefile.global.in |   21 ++++++++++++++++++---
src/makefiles/pgxs.mk  |   15 ---------------
2 files changed, 18 insertions(+), 18 deletions(-)