ci: fix warnings and enable --werror for the mingw build

Fix various warnings regarding unused variables, duplicated
branches etc by adjusting the ifdeffery and some missing casts.

gnulib triggers -Wduplicated-branches in one of the copied files,
disable as that just makes updating the code harder.

The warning indicating missing features are made none fatal through
pragmas. They still show but don't abort the build.

https://bugzilla.gnome.org/show_bug.cgi?id=793729

gio/glocalfile.c

@@ -2819,7 +2819,9 @@ g_local_file_measure_size_of_file (gint           parent_fd,
   if (S_ISDIR (buf.st_mode))
     {
       int dir_fd = -1;
+#ifdef AT_FDCWD
       int errsv;
+#endif
 
       if (g_cancellable_set_error_if_cancelled (state->cancellable, error))
         return FALSE;