aclocal: diagnose non-existing directories in AC_CONFIG_MACRO_DIRS better

aclocal: diagnose non-existing directories in AC_CONFIG_MACRO_DIRS better

This new implementation ensures that any directory (possibly excluding
the first one, if the '--install' option is used) that is declared with
AC_CONFIG_MACRO_DIRS and that is non-existent will cause an error from
aclocal.

* aclocal.in (scan_m4_dirs): Add a new argument, telling whether it's OK
for the scanned directory to be non-existing.  Adjust the implementation
accordingly.
($first_user_m4dir): Remove, no more needed.
(scan_m4_files): Update 'scan_m4_dirs' invocations so that aclocal will
not complain if the first user macro directory is non-existing and the
'--install' option is given: such directory will be created later by
aclocal itself.
* t/aclocal-macrodir.tap: Do not mark the last test as TODO anymore;
it now passes.  Make stricter by ensuring a non-existing directory in
AC_CONFIG_MACRO_DIRS causes an hard error, not a warning.

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