build: require GNU make in order to run the maintainer checks

build: require GNU make in order to run the maintainer checks

This is not a real regression in portability, since most maintainer
check rules already assumed GNU grep, and were anyway only meant to
run on the developers' systems (where we can safely assume the
presence of GNU make).

This change will allow us to take advantage, in future changes, of
more GNU make features, thus simplifying or optimizing some of our
maintainer rules.

* GNUmakefile: New, including 'Makefile' and 'syntax-checks.mk'.
* configure.ac (AC_CONFIG_LINKS): Link it in the builddir.
* Makefile.am (EXTRA_DIST): Distribute 'GNUmakefile' and
'syntax-checks.mk'.
Move all syntax-check rules and auxiliary variables into ...
* syntax-checks.mk: ... this new file, with some adjustments.