Run 'gettextize -f --no-changelog' to update to 0.21.

This removes the entire intl directory, and relies on an external
libintl only.

$(INTLLIBS) should now be referred to as $(LIBINTL), so change all
references.

Unfortunately, some of the gettext m4 files were modified and "mutt
customized" in the past.  I'm trying to keep them in a pristine state
after this upgrade.  glibc21.m4, lcmessage.m4, codeset.m4 have also
been removed.

Manually call AM_ICONV in configure.ac.  This was previously called
in the modified "MUTT_AM_GNU_GETTEXT" but the result $am_cv_func_iconv
is tested just below.

locale.h is a standard library defined by C99.  The gpgme and pgp code
wrapped those includes in HAVE_LOCALE_H checks, but that check was
performed inside of gettext.m4.  Remove the check inside those
files (locale.h is already included unconditionally elsewhere in
Mutt).

Copy the sample gettext.h file and include that instead, as suggested
by the gettext documentation.

unistd.h is al...