intl: Fix list of exported symbols (regression 2021-06-19).

intl: Fix list of exported symbols (regression 2021-06-19).

Reported by <fxcoudert@gcc.gnu.org>
and Christian Weisgerber <naddy@mips.inka.de>
in <https://savannah.gnu.org/bugs/?64323>
and <https://lists.gnu.org/archive/html/bug-gettext/2023-07/msg00005.html>.
Analyzed by Christian Weisgerber <naddy@mips.inka.de> and
Tijl Coosemans <tijl@FreeBSD.org>.

* gettext-runtime/intl/Makefile.am (AM_CPPFLAGS): Set BUILDING_DLL.
* gettext-runtime/intl/intl-compat.c: Update comments.