Improve detection of GNU make, avoiding "Arg list too long" errors.

Improve detection of GNU make, avoiding "Arg list too long" errors.

Such errors could take place when the main makefile included too many
sub-makefiles, making $(MAKEFILE_LIST) too long and causing the
recipes $(am__is_gnu_make) to exceed the shell's command-line length
limits.  This is not a theoretical issue: it could happen for projects
having lots of C/C++ sources and using automatic dependency tracking,
which created an included .Po sub-makefile for each of such sources.

Fixes http://debbugs.gnu.org/18744

* lib/am/header-vars.am (am__is_gnu_make): Fix the logic to avoid
the use of $(MAKEFILE_LIST).
* NEWS: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>