Merge pull request #179 from yrashk/patch-1

Problem: impossible to supply custom PG_CONFIG to PGXS

Author: okbob

Makefile

@@ -20,7 +20,7 @@ top_builddir = ../..
 include $(top_builddir)/src/Makefile.global
 include $(top_srcdir)/contrib/contrib-global.mk
 else
-PG_CONFIG = pg_config
+PG_CONFIG ?= pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
 include $(PGXS)
 endif