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.

src/Makefile.global.in

@@ -40,9 +40,24 @@ VERSION = @PACKAGE_VERSION@
 MAJORVERSION = @PG_MAJORVERSION@
 VERSION_NUM = @PG_VERSION_NUM@
 
-# Support for VPATH builds
-# (PGXS VPATH support is handled separately in pgxs.mk)
-ifndef PGXS
+# Set top_srcdir, srcdir, and VPATH.
+ifdef PGXS
+top_srcdir = $(top_builddir)
+
+# If VPATH is set or Makefile is not in current directory we are building
+# the extension with VPATH so we set the variable here.
+ifdef VPATH
+srcdir = $(VPATH)
+else
+ifeq ($(CURDIR),$(dir $(firstword $(MAKEFILE_LIST))))
+srcdir = .
+VPATH =
+else
+srcdir = $(dir $(firstword $(MAKEFILE_LIST)))
+VPATH = $(srcdir)
+endif
+endif
+else # not PGXS
 vpath_build = @vpath_build@
 abs_top_builddir = @abs_top_builddir@
 abs_top_srcdir = @abs_top_srcdir@

src/makefiles/pgxs.mk

@@ -62,21 +62,6 @@ ifdef PGXS
 top_builddir := $(dir $(PGXS))../..
 include $(top_builddir)/src/Makefile.global
 
-top_srcdir = $(top_builddir)
-# If VPATH is set or Makefile is not in current directory we are building
-# the extension with VPATH so we set the variable here.
-ifdef VPATH
-srcdir = $(VPATH)
-else
-ifeq ($(CURDIR),$(dir $(firstword $(MAKEFILE_LIST))))
-srcdir = .
-VPATH =
-else
-srcdir = $(dir $(firstword $(MAKEFILE_LIST)))
-VPATH = $(srcdir)
-endif
-endif
-
 # These might be set in Makefile.global, but if they were not found
 # during the build of PostgreSQL, supply default values so that users
 # of pgxs can use the variables.