BUG #15407: [minor] build depends on $MAKELEVEL being 0 at topMakefile

The following bug has been logged on the website:

Bug reference:      15407
Logged by:          Torsten Lüttgert
Email address:      st-postgresql@luettgert.de
PostgreSQL version: 11beta4
Operating system:   CentOS 7
Description:

We build postgresql as part of a larger runtime environment, and if the
top-level make is invoked from an existing, other Makefile (i.e., with
$MAKELEVEL set), the build fails like this:

relpath.c:21:37: fatal error: catalog/pg_tablespace_d.h: No such file or
directory
 #include "catalog/pg_tablespace_d.h"

The file is there:
# find -name pg_tablespace_d.h
./src/backend/catalog/pg_tablespace_d.h

Workaround for us was to use
  MAKELEVEL=0 $(MAKE)
instead.