meson: ignore -Dsmack-run-label= if -Dsmack=false

Compilation would fail because we could have HAVE_SMACK_RUN_LABEL without
HAVE_SMACK. This doesn't make much sense, so let's just make -Dsmack=false
completely disable smack.

Also, the logic in smack-setup.c seems dubious: '#ifdef SMACK_RUN_LABEL'
would evaluate to true even if -Dsmack-run-label='' is used. I think
this was introduced in the conversion to meson:
8b197c3a8a added

AC_ARG_WITH(smack-run-label,
AS_HELP_STRING([--with-smack-run-label=STRING],
        [run systemd --system with a specific SMACK label]),
        [AC_DEFINE_UNQUOTED(SMACK_RUN_LABEL, ["$withval"], [Run with a smack label])],
        [])

i.e. it really was undefined if not specified. And it was same
still in 72cdb3e783 when configure.ac
was dropped.

So let's use the single conditional HAVE_SMACK_RUN_LABEL everywhere.

meson.build

@@ -1024,8 +1024,11 @@ else
 endif
 conf.set10('HAVE_APPARMOR', have)
 
-conf.set10('HAVE_SMACK_RUN_LABEL', get_option('smack-run-label') != '')
-conf.set_quoted('SMACK_RUN_LABEL', get_option('smack-run-label'))
+have = get_option('smack') and get_option('smack-run-label') != ''
+conf.set10('HAVE_SMACK_RUN_LABEL', have)
+if have
+        conf.set_quoted('SMACK_RUN_LABEL', get_option('smack-run-label'))
+endif
 
 want_polkit = get_option('polkit')
 install_polkit = false

src/core/smack-setup.c

@@ -322,7 +322,7 @@ int mac_smack_setup(bool *loaded_policy) {
                 return 0;
         }
 
-#ifdef SMACK_RUN_LABEL
+#if HAVE_SMACK_RUN_LABEL
         r = write_string_file("/proc/self/attr/current", SMACK_RUN_LABEL, WRITE_STRING_FILE_DISABLE_BUFFER);
         if (r < 0)
                 log_warning_errno(r, "Failed to set SMACK label \"" SMACK_RUN_LABEL "\" on self: %m");

src/shared/mount-setup.c

@@ -278,7 +278,7 @@ static int symlink_controller(const char *target, const char *alias) {
         if (r < 0)
                 return log_error_errno(r, "Failed to create symlink %s: %m", a);
 
-#ifdef SMACK_RUN_LABEL
+#if HAVE_SMACK_RUN_LABEL
         const char *p;
 
         p = strjoina("/sys/fs/cgroup/", target);