Make numeric form of PG version number readily available in Makefiles.

Expose PG_VERSION_NUM (e.g., "90600") as a Make variable; but for
consistency with the other Make variables holding similar info,
call the variable just VERSION_NUM not PG_VERSION_NUM.

There was some discussion of making this value available as a pg_config
value as well.  However, that would entail substantially more work than
this two-line patch.  Given that there was not exactly universal consensus
that we need this at all, let's just do a minimal amount of work for now.

Back-patch of commit a5d489ccb7, so that this
variable is actually useful for its intended purpose sometime before 2020.

Michael Paquier, reviewed by Pavel Stehule

configure

@@ -642,6 +642,7 @@ ac_includes_default="\
 
 ac_subst_vars='LTLIBOBJS
 vpath_build
+PG_VERSION_NUM
 OSX
 XSLTPROC
 COLLATEINDEX
@@ -29407,6 +29408,7 @@ _ACEOF
 
 
 
+
 # prepare build tree if outside source tree
 # Note 1: test -ef might not exist, but it's more reliable than `pwd`.
 # Note 2: /bin/pwd might be better than shell's built-in at getting