Don't use -lc explicitly when linking with libtool.
libtool has logic that determines whether it should add -lc, and this logic
works fine on all modern platforms.
Reported by Michael Forney <mforney@mforney.org>
at <https://savannah.gnu.org/bugs/?40192>.
* gettext-runtime/m4/intl.m4 (AM_INTL_SUBDIR): Don't set LTLIBC.
* gettext-runtime/intl/Makefile.in (libintl.la, libgnuintl.la): Don't use
LTLIBC.
