m4: Rely less on internal symbols

m4: Rely less on internal symbols

* gettext-runtime/m4/gettext.m4 (AM_GNU_GETTEXT): Skip checks for the
internal symbols _nl_msg_cat_cntr, _nl_domain_bindings, and
_nl_expand_alias, if __GNU_GETTEXT_SUPPORTED_REVISION is defined.
Problem reported and fix suggested by Masanori Ogino in:
https://lists.gnu.org/archive/html/bug-gettext/2016-04/msg00000.html
The rationale behind this is: (1) those symbol checks are for detecting
certain broken implementations, namely NetBSD and Solaris 7, and
(2) __GNU_GETTEXT_SUPPORTED_REVISION is not defined in those
implementations, while it is defined on compatible implementations, such
as musl-libc which doesn't have those internal symbols.