vala: configure exit with status 77, not 1, if valac version is too old

vala: configure exit with status 77, not 1, if valac version is too old

From a report by Bruno Haible in automake bug#1193.  This change should
also automatically avoid spurious testsuite failures with older vala
versions.

* m4/vala.m4 (AM_PROG_VALAC): Exit with status 77, rather than 1, if
the Vala compiler found older than the minimal required version (if
any).  This is more consistent with what is done by other macros like
AM_PROG_UPC or AC_PROG_CC.
* NEWS: Update.
* t/vala-headers.sh: Adjust, so that the test is only skipped of the
vala compiler is too old or the required PKG_CHECK_MODULES third-party
macro is not found by aclocal, and not if a generic error happens in
the configure script.
* t/vala-libs.sh: Likewise.
* t/vala2.sh: Likewise.
* t/vala3.sh: Likewise.
* t/vala5.sh: Likewise.
* t/vala-vpath.sh: Likewise (but don't check for the potential error
with PKG_CHECK_MODULES, as that macro is not used in any way here).
* t/vala4.sh: Adjust, to avoid the new semantic causing this test to
skip instead of failing.

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