Explicitly discard unused results where necessary

setre*id() functions are declared with `warn_unused_result` which makes
them emit a warning when their result is unused. This warning can be
suppressed by using (void) cast in clang, so do this where appropriate.
As gcc doesn't support this, disable the warning completely when gcc is
used.

See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425

meson.build

@@ -119,6 +119,14 @@ compiler_c_flags += cc.get_supported_arguments([
   '-Wno-declaration-after-statement',
 ])
 
+if cc.get_id() == 'gcc'
+  compiler_c_flags += cc.get_supported_arguments([
+    # -Wunused-result can't be suppressed by using (void) in gcc, so let's disable it for now
+    # See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425
+    '-Wno-unused-result',
+  ])
+endif
+
 glib_req_version = '>= 2.30.0'
 
 gio_dep = dependency('gio-2.0', version: glib_req_version)

src/polkitbackend/polkitd.c

@@ -173,8 +173,8 @@ become_user (const gchar  *user,
       goto out;
     }
 
-  setregid (pw->pw_gid, pw->pw_gid);
-  setreuid (pw->pw_uid, pw->pw_uid);
+  (void) setregid (pw->pw_gid, pw->pw_gid);
+  (void) setreuid (pw->pw_uid, pw->pw_uid);
   if ((geteuid () != pw->pw_uid) || (getuid () != pw->pw_uid) ||
       (getegid () != pw->pw_gid) || (getgid () != pw->pw_gid))
     {

src/programs/pkexec.c

@@ -968,7 +968,7 @@ main (int argc, char *argv[])
   /* if not changing to uid 0, become uid 0 before changing to the user */
   if (pw->pw_uid != 0)
     {
-      setreuid (0, 0);
+      (void) setreuid (0, 0);
       if ((geteuid () != 0) || (getuid () != 0))
         {
           g_printerr ("Error becoming uid 0: %s\n", g_strerror (errno));
@@ -1021,8 +1021,8 @@ main (int argc, char *argv[])
       g_printerr ("Error initializing groups for %s: %s\n", pw->pw_name, g_strerror (errno));
       goto out;
     }
-  setregid (pw->pw_gid, pw->pw_gid);
-  setreuid (pw->pw_uid, pw->pw_uid);
+  (void) setregid (pw->pw_gid, pw->pw_gid);
+  (void) setreuid (pw->pw_uid, pw->pw_uid);
   if ((geteuid () != pw->pw_uid) || (getuid () != pw->pw_uid) ||
       (getegid () != pw->pw_gid) || (getgid () != pw->pw_gid))
     {