Fix for PR automake/381:

Fix for PR automake/381:
* automake.in (handle_gettext): Do not print diagnostics
about po/ and intl/ missing from SUBDIRS if po/ does not
exist.  Warn if `SUBDIRS = intl' is used although libintl
is 'external'.
* tests/gettext.test: Make sure diagnostics are not output
when po/ does not exist.
* tests/gettext2.test: Test for unwanted intl/.
PR from Alexander Turbov, fix suggested by Bruno Haible.