libc: fix using a custom allocator.

Author: adriweb

src/libc/allocator.src

@@ -32,22 +32,29 @@ if defined ALLOCATOR_SIMPLE
 _malloc := __simple_malloc
 _free := __simple_free
 _realloc := __simple_realloc
+	extern  __simple_free
+	extern  __simple_malloc
+	extern  __simple_realloc
 
-end if
-
-if defined ALLOCATOR_STANDARD
+else if defined ALLOCATOR_STANDARD
 
 _malloc := __standard_malloc
 _free := __standard_free
 _realloc := __standard_realloc
-
-end if
-
-	extern  __simple_free
-	extern  __simple_malloc
-	extern  __simple_realloc
 	extern  __standard_malloc
 	extern  __standard_free
 	extern  __standard_realloc
+
+else ; custom functions provided by the program
+
+_malloc := __custom_malloc
+_free := __custom_free
+_realloc := __custom_realloc
+	extern  __custom_malloc
+	extern  __custom_free
+	extern  __custom_realloc
+
+end if
+
 	extern	__imulu
 	extern	_memset