tests: can check our recipes avoid trailing backslashes

tests: can check our recipes avoid trailing backslashes

This is related to commit v1.11-1704-g254227b of 2012-05-01,
"parallel-tests: avoid trailing backslashes in make recipes",
and automake bug#10436.

Recipes with a trailing backslash character (possibly followed by
blank characters only) can cause spurious syntax errors with at
least older bash versions (e.g., bash 2.05b), and can be potentially
be unportable to other weaker shells.

So provide a target that runs the testsuite looking for this kind
of breakage (without requiring a real bugged shell).

* t/ax/shell-no-trail-bslash.in: New, a "shell" that chokes on '-c'
commands having a trailing '\' (possibly followed by whitespace only).
* Makefile (t/ax/shell-no-trail-bslash): Generate this script from it.
(noinst_SCRIPTS, CLEANFILES): Add it.
(EXTRA_DIST): Add 't/ax/shell-no-trail-bslash.in'.
(check-no-trailing-backslash-in-recipes): New target, runs the testsuite
with 'shell-no-trail-bslash' as the CONFIG_SHELL, to catch possible
recipes having a trailing backslash character (possibly followed by
* .gitignore: Update.
* t/self-check-shell-no-trail-bslash.sh: New testsuite self-check.
* t/parallel-tests-trailing-bslash.sh: Remove as obsolete.
* t/list-of-tests.mk: Adjust.

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