pgsql: Add PG_CFLAGS, PG_CXXFLAGS, and PG_LDFLAGS variables to PGXS

Add PG_CFLAGS, PG_CXXFLAGS, and PG_LDFLAGS variables to PGXS

Add PG_CFLAGS, PG_CXXFLAGS, and PG_LDFLAGS variables to pgxs.mk which
will be appended or prepended to the corresponding make variables.
Notably, there was previously no way to pass custom CXXFLAGS to third
party extension module builds, COPT and PROFILE supporting only CFLAGS
and LDFLAGS.

Backpatch all the way down to ease integration with existing
extensions.

Author: Christoph Berg
Reviewed-by: Andres Freund, Tom Lane, Michael Paquier
Discussion: https://postgr.es/m/20181113104005.GA32154@msg.credativ.de
Backpatch-through: 9.4

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/5e63df82708ba6e350a1beb568c059e109f440a0

Modified Files
--------------
doc/src/sgml/extend.sgml | 29 ++++++++++++++++++++++++++++-
src/makefiles/pgxs.mk | 14 +++++++++++++-
2 files changed, 41 insertions(+), 2 deletions(-)