core: Make libmount optional

Instead of skipping libcore entirely when libmount is not available,
let's only compile out the pieces that need libmount. This makes the
meson logic much less complex and allows systemd-analyze to be built
when libmount is not available.

src/shared/libmount-util.h

@@ -70,6 +70,15 @@ int libmount_parse_fstab(struct libmnt_table **ret_table, struct libmnt_iter **r
 int libmount_is_leaf(
                 struct libmnt_table *table,
                 struct libmnt_fs *fs);
+#else
+
+struct libmnt_monitor;
+
+static inline void *sym_mnt_unref_monitor(struct libmnt_monitor *p) {
+        assert(p == NULL);
+        return NULL;
+}
+
 #endif
 
 int dlopen_libmount(void);