glibc: restore buildability in musl masterdirs

It's generally unsupported, but glibc builds fine in a musl masterdir,
so we might as well check if localedef is available when building the
C.UTF-8 locale and proceed with a warning if not.

srcpkgs/glibc/template

@@ -136,7 +136,7 @@ do_install() {
 	if [ -z "${CROSS_BUILD}" ]; then
 		make -C localedata DESTDIR="${DESTDIR}" \
 			objdir="../build" install-files-C.UTF-8/UTF-8
-	else
+	elif command -v localedef >/dev/null 2>&1; then
 		local endian=
 		case "$XBPS_TARGET_ENDIAN" in
 			be) endian="--big-endian" ;;
@@ -146,6 +146,8 @@ do_install() {
 		vmkdir usr/lib/locale
 		I18NPATH=./localedata localedef --prefix="${DESTDIR}" ${endian} \
 			-i C -f UTF-8 --no-archive "${DESTDIR}/usr/lib/locale/C.utf8"
+	else
+		msg_warn "localedef not available; skipping generation of C.UTF-8 locale\n"
 	fi
 
 	# create /etc/default/libc-locales