[ng] refactor: consider $(EXEEXT) used if non-empty at make runtime

[ng] refactor: consider $(EXEEXT) used if non-empty at make runtime

To check whether we should handle $(EXEEXT), simply check for
"ifdef EXEEXT" in our Makefile fragments, rather than also checking
whether EXEEXT has been set as a configure-time AC_SUBST.  This
simplifies the logic, and move it completely at make runtime, rather
than having it split between automake runtime (to decide whether
EXEEXT is AC_SUBST) and make runtime (to decide whether $(EXEEXT) is
empty).

* lib/am/header-vars.am (EXEEXT): Default to empty.  This is required
to avoid environment interferences.
(am.conf.handle-exeext): Define this no more.
* lib/am/parallel-tests.am, lib/am/serial-tests.am: Simply look
at $(EXEEXT), rather than using $(am.conf.handle-exeext), or the
'%?HANDLE-EXEEXT%' transform.
* automake.in (process_file): Don't define that transform anymore.

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