Document need to clear MAKELEVEL when invoking PG build from a makefile.

Document need to clear MAKELEVEL when invoking PG build from a makefile.

Since commit 3b8f6e75f, failure to do this would lead to
submake-generated-headers not doing anything, so that references to
generated or symlinked headers would fail.  Previous to that, the
omission only led to temp-install not doing anything, which apparently
affects many fewer people (doesn't anybody use "make check" in their
build rules??).  Hence, backpatch to v11 but not further.

Per complaints from Christoph Berg, Jakob Egger, and others.