Allow compiling with custom allocators

User can define hb_malloc_impl, etc, to name of custom allocator functions
that have the same signature as malloc.

src/hb-private.hh

@@ -54,6 +54,23 @@
 #include <stdarg.h>
 
 
+/* Compile-time custom allocator support. */
+
+#if defined(hb_malloc_impl) \
+ && defined(hb_calloc_impl) \
+ && defined(hb_realloc_impl) \
+ && defined(hb_free_impl)
+extern void* hb_malloc_impl(size_t size);
+extern void* hb_calloc_impl(size_t nmemb, size_t size);
+extern void* hb_realloc_impl(void *ptr, size_t size);
+extern void  hb_free_impl(void *ptr);
+#define malloc hb_malloc_impl
+#define calloc hb_calloc_impl
+#define realloc hb_realloc_impl
+#define free hb_free_impl
+#endif
+
+
 /* Compiler attributes */