locale-util: dlopen() libintl instead of linking against it

dgettext() lives in libc on glibc and in libintl.so.8 on musl with
gettext. Resolve it via dlsym() so neither configuration produces a
hard link-time dependency on libintl: try libintl.so.8 first and fall
back to RTLD_DEFAULT (which finds dgettext in libc on glibc).

The _() macro now expands to a runtime check that returns the
untranslated string if dlopen_libintl() has not run successfully, so
callers don't have to gate every translatable message on a runtime
check. pam_systemd_home — currently the only consumer of _() — calls
dlopen_libintl() best-effort from each PAM entry point.

The meson find_library('intl') dance is replaced with a has_header()
check; the only thing we need at build time is the prototype.

src/basic/locale-util.c

@@ -7,7 +7,12 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
+#ifndef __GLIBC__
+#include "sd-dlopen.h"
+#endif
+
 #include "dirent-util.h"
+#include "dlfcn-util.h"
 #include "env-util.h"
 #include "fd-util.h"
 #include "fileio.h"
@@ -22,6 +27,32 @@
 #include "strv.h"
 #include "utf8.h"
 
+#ifdef __GLIBC__
+DLSYM_PROTOTYPE(dgettext) = dgettext;
+#else
+DLSYM_PROTOTYPE(dgettext) = NULL;
+#endif
+
+int dlopen_libintl(int log_level) {
+#ifdef __GLIBC__
+        return 1;
+#else
+        static void *libintl_dl = NULL;
+
+        SD_ELF_NOTE_DLOPEN(
+                        "intl",
+                        "Support for message translation via gettext",
+                        SD_ELF_NOTE_DLOPEN_PRIORITY_SUGGESTED,
+                        "libintl.so.8");
+
+        return dlopen_many_sym_or_warn(
+                        &libintl_dl,
+                        "libintl.so.8",
+                        log_level,
+                        DLSYM_ARG(dgettext));
+#endif
+}
+
 static char* normalize_locale(const char *name) {
         const char *e;
 

src/basic/locale-util.h

@@ -1,9 +1,18 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <libintl.h>    /* IWYU pragma: export */
 #include <locale.h>     /* IWYU pragma: export */
 
 #include "basic-forward.h"
+#include "dlfcn-util.h"
+
+/* format_arg(2) propagates the format-string nature of the second argument to the return value, so that
+ * printf(_("Hello %s"), name) still gets checked. It survives both DLSYM_PROTOTYPE's typeof() and the
+ * ternary in _() below — verified on gcc and clang. */
+extern DLSYM_PROTOTYPE(dgettext) __attribute__((format_arg(2)));
+
+int dlopen_libintl(int log_level);
 
 typedef enum LocaleVariable {
         /* We don't list LC_ALL here on purpose. People should be
@@ -31,7 +40,9 @@ int get_locales(char ***ret);
 bool locale_is_valid(const char *name);
 int locale_is_installed(const char *name);
 
-#define _(String) dgettext(GETTEXT_PACKAGE, String)
+/* Falls back to the untranslated string if dlopen_libintl() hasn't run or has failed, so callers don't have
+ * to gate every translatable message on a runtime check. */
+#define _(String) (sym_dgettext ? sym_dgettext(GETTEXT_PACKAGE, (String)) : (String))
 #define N_(String) String
 
 bool is_locale_utf8(void);