Support custom memory management macros

Users can define their custom memory management macros.

NOTE: Most external libraries support custom macros in the same way, raylib should redefine those macros to raylib ones, to unify custom memory loading. That redefinition is only implemented as example for stb_image.h in [textures] module.

src/raudio.h

@@ -56,7 +56,16 @@
 //----------------------------------------------------------------------------------
 // Defines and Macros
 //----------------------------------------------------------------------------------
-//...
+// Allow custom memory allocators
+#ifndef RL_MALLOC
+    #define RL_MALLOC(sz)       malloc(sz)
+#endif
+#ifndef RL_CALLOC
+    #define RL_CALLOC(n,sz)     calloc(n,sz)
+#endif
+#ifndef RL_FREE
+    #define RL_FREE(p)          free(p)
+#endif
 
 //----------------------------------------------------------------------------------
 // Types and Structures Definition