Build: changes needed to build on FreeBSD

* Fix SDL not finding Xlib.h
* Link to clangSupport library for newer LLVM versions
* Add FreeBSD essential symbols to symbols_unix.map

Pull Request: blender/blender#105892

source/creator/symbols_unix.map

@@ -39,6 +39,9 @@ global:
   realpath;
   sched_*;
   valloc;
+  /* needed on FreeBSD */
+  __progname;
+  environ;
 local:
   *;
 };