match glibc in ConditionVersion

Extend ConditionVersion= to allow matching against glibc version,
as proposed in https://github.com/systemd/systemd/pull/36468#issuecomment-2674600909

src/shared/condition.c

@@ -4,6 +4,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <fnmatch.h>
+#include <gnu/libc-version.h>
 #include <limits.h>
 #include <stdlib.h>
 #include <sys/stat.h>
@@ -258,6 +259,9 @@ static int condition_test_version(Condition *c, char **env) {
         if (streq(word, "systemd"))
                 return condition_test_version_cmp(p, STRINGIFY(PROJECT_VERSION));
 
+        if (streq(word, "glibc"))
+                return condition_test_version_cmp(p, gnu_get_libc_version());
+
         /* if no predicate has been set, default to "kernel" and use the whole parameter as condition */
         if (!streq(word, "kernel"))
                 p = c->parameter;