GNUmakefile: reenable "make syntax-check" for most projects

GNUmakefile: reenable "make syntax-check" for most projects

Since Friday's commit 05e2d798, "maint.mk: don't maintain a second
build-aux variable", "syntax-check" would do nothing but succeed with
the "No version control files detected..." diagnostic (unless you
happened to override _build-aux via cfg.mk).
* top/GNUmakefile (_autoreconf, _build-aux): Move default definitions
to precede inclusion of maint.mk.  Otherwise, these variables would
be used undefined in any project that does not override the default.