general: include verbatim makefile fragments in output Makefiles

general: include verbatim makefile fragments in output Makefiles

Instead of copying their contents in each output Makefile.

* automake.in (verbatim): Instead of copying the given Makefile fragment
in the output makefile, copy it in the '.mk' subdirectory of the top-level
source directory (i.e., the one automake is being run into).  And return
a proper "include" directive that will allow GNU make to include that
fragment in the generated Makefile at make runtime.
* lib/am/distdir.am: Distribute the '.mk' subdirectory of the top-level
source directory, with the help of ...
(am.dist.mk-files): ... this new internal variable.
* .gitignore: Add the '.mk' directory.
* Makefile.am (amhello_configury): Likewise.
* t/all2.sh: Adjust.
* t/ax/depcomp.sh: Likewise.
* t/ax/tap-setup.sh: Likewise.
* t/check.sh: Likewise.
* t/distdir.sh: Likewise.
* t/installdir.sh: Likewise.
* t/noinst.sh: Likewise.
* t/phony.sh: Likewise.
* t/spell.sh: Likewise.
* t/spell2.sh: Likewise.
* t/conflnk2.sh: Adjust, and refactor a bit while we are at it.
* t/serial-tests.sh: Likewise (greatly simplifying it in the process).
* t/txinfo-rules-once.sh: Remove: its grepping tests has been rendered
almost all obsolete, and it would be to time-consuming to try to update
them.
* t/dejagnu.sh: Remove as obsolete.
* t/instexec.sh: Likewise.
* t/clean.sh: Likewise.

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