maint: no more make recursion in Automake's build system

maint: no more make recursion in Automake's build system

We finally merge testsuite-related part of the Automake build
system with the top-level one.  See also yesterday's commit
'v1.11-2124-ga4b0f4b', "maint: remove most recursion in automake's
own build system".

* t/gen-testsuite-part: Move ...
* gen-testsuite-part: ... here, with required adjustments.
* t/defs-static.in: Move ...
* defs-static.in: ... here, with required adjustments.  In
particular ...
($testsrcdir, $testbuilddir): Removed.
($top_testsrcdir): Renamed ...
($am_top_srcdir): ... to this.
($top_testbuilddir): Renamed ...
($am_top_builddir): ... to this.
* t/defs: Move ...
* defs: ... here, and adjust as required.
* t/Makefile.am: Merge ...
* Makefile.am: ... in here, with related adjustments and
simplifications.
* bootstrap: Adjust as required.
* syntax-checks.mk: Likewise.
* .gitignore: Likewise.
* t/ax/tap-setup.sh: Likewise.
* t/get-sysconf.sh: Likewise.
* t/help-multilib.sh: Likewise.
* t/multlib.sh: Likewise.
* t/parallel-tests2.sh: Likewise.
* t/self-check-cleanup.tap: Likewise.
* t/self-check-exit.tap: Likewise.
* t/self-check-dir.tap: Likewise.
* t/self-check-explicit-skips.sh: Likewise.
* t/self-check-reexec.tap: Likewise.
* t/self-check-sanity.sh: Likewise.
* t/yacc-d-cxx.sh: Likewise.

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