Customize Lua GC, add object-memory-lock API.

Add custom API to prevent Lua Objects from being garbage collected.
This is intended to for Ardour LuaBridge bindings (~1MB Objects:
tables, functions and userdata).
The bindings are persistent and the gc can skip them in mark & sweep
phases. This is a significant performance improvement for garbage
collection.

Note. The next version of Lua (5.4) will come with a generational-gc
rather than an incremental, so extending the API at this point in time
is acceptable.

libs/lua/lua-5.3.4/lstate.h

@@ -144,6 +144,7 @@ typedef struct global_State {
   unsigned int gcfinnum;  /* number of finalizers to call in each GC step */
   int gcpause;  /* size of pause between successive GCs */
   int gcstepmul;  /* GC 'granularity' */
+  int gcmlock;  /* memory lock new objects - fixedgc */
   lua_CFunction panic;  /* to be called in unprotected errors */
   struct lua_State *mainthread;
   const lua_Number *version;  /* pointer to version number */