Fix PGXS vpath build when PostgreSQL is built with vpath

PGXS computes srcdir from VPATH, PostgreSQL proper computes VPATH from
srcdir, and doing both results in an error from make.  Conditionalize so
only one of these takes effect.

src/Makefile.global.in

@@ -40,6 +40,8 @@ VERSION = @PACKAGE_VERSION@
 MAJORVERSION = @PG_MAJORVERSION@
 
 # Support for VPATH builds
+# (PGXS VPATH support is handled separately in pgxs.mk)
+ifndef PGXS
 vpath_build = @vpath_build@
 abs_top_srcdir = @abs_top_srcdir@
 
@@ -51,6 +53,7 @@ top_srcdir = $(abs_top_srcdir)
 srcdir = $(top_srcdir)/$(subdir)
 VPATH = $(srcdir)
 endif
+endif # not PGXS
 
 vpathsearch = `for f in $(addsuffix /$(1),$(subst :, ,. $(VPATH))); do test -r $$f && echo $$f && break; done`