Added gcc option '-Wconversion'

No warnings for standard numerical types. Still pending alternative
numerical types.

lmem.h

@@ -85,7 +85,7 @@ LUAI_FUNC void *luaM_saferealloc_ (lua_State *L, void *block, size_t oldsize,
                                                               size_t size);
 LUAI_FUNC void luaM_free_ (lua_State *L, void *block, size_t osize);
 LUAI_FUNC void *luaM_growaux_ (lua_State *L, void *block, int nelems,
-                               int *size, int size_elem, int limit,
+                               int *size, unsigned size_elem, int limit,
                                const char *what);
 LUAI_FUNC void *luaM_shrinkvector_ (lua_State *L, void *block, int *nelem,
                                     int final_n, int size_elem);