autoconf: #include <libintl.h> when checking for gettext()

autoconf: #include <libintl.h> when checking for gettext()

Some libc implementations like uclibc or musl provides
gettext stubs via libintl library but this case is not checked
by AC_CHECK_LIB(c, gettext ...) because gcc has gettext as builtin
which passess the check.

So check it with included libintl.h where gettext may unfold into
libintl_gettext which will cause check to fail if libintl_gettext are
needed to be linked with -lintl.

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>