tests: improve 'pkg-config' requirement

tests: improve 'pkg-config' requirement

This improves (and unifies among different tests) the detection of
both the 'pkg-config' program and m4 macros provided by it; they are
needed in particular by tests on Vala support.

Before this, some Vala-related tests were failing spuriously when the
pkg-config program was available but the macros from pkg.m4 were not
made explicitly available in ACLOCAL_PATH.  These tests were:

    vala-mix vala-mix2 vala-parallel vala-vapi vala-vpath

See also similar commit 'v1.11-581-gb7d67d5' of 2011-12-22,
"tests: better handling of gettext and libtool requirements".

* t/ax/am-test-lib.sh (process_requirements): If a requirement
matches the 'pkg-config*' wildcard, try to make available to it
the pkg-config m4 macros, and skip it if it's not possible.
This is done by sourcing the 'get.sh' fragment generated by ...
* t/pkg-config-macros.sh: ... this new test, which looks in
advance for the 'pkg-config' program and the m4 macros that
comes with it.
* t/list-of-tests.mk: Add the new test.
* gen-testsuite-part (%deps_extractor): Tests requiring pkg-config
will need the m4 macros coming with it; so these tests should be
run after the 'pkg-config-macros.sh' test.  Automatically generate
proper declarations for such dependencies.
* t/vala-header.sh: No need to explicitly check that the macro
PKG_CHECK_MODULES has been expanded by autoconf.
* t/vala-libs.sh: Likewise.
* t/vala-non-recursive-setup.sh: Likewise.
* t/vala-per-target-flags.sh: Likewise.
* t/vala-recursive-setup.sh: Likewise.

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