Remove USE_VPATH make variable from PGXS

The user can just set VPATH directly.  There is no need to invent
another variable.

Author: petere

doc/src/sgml/extend.sgml

@@ -1180,10 +1180,10 @@ make -f /path/to/extension/source/tree/Makefile install
     way to how it is done for the core code. One way to do this is using the
     core script <filename>config/prep_buildtree</>. Once this has been done
     you can build by setting the <literal>make</literal> variable
-    <varname>USE_VPATH</varname> like this:
+    <varname>VPATH</varname> like this:
 <programlisting>
-make USE_VPATH=/path/to/extension/source/tree
-make USE_VPATH=/path/to/extension/source/tree install
+make VPATH=/path/to/extension/source/tree
+make VPATH=/path/to/extension/source/tree install
 </programlisting>
     This procedure can work with a greater variety of directory layouts.
    </para>

src/makefiles/pgxs.mk

@@ -63,11 +63,10 @@ top_builddir := $(dir $(PGXS))../..
 include $(top_builddir)/src/Makefile.global
 
 top_srcdir = $(top_builddir)
-# If USE_VPATH is set or Makefile is not in current directory we are building
-# the extension with VPATH so we set the variable here
-ifdef USE_VPATH
-srcdir = $(USE_VPATH)
-VPATH = $(USE_VPATH)
+# 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 = .