Make a brk/sbrk emulation.

Mac OS X seems to have some difficulties with brk/sbrk (maybe with the
4MB heap limit), and replace all the allocation logic will be prone to
errors, I'll add a new define and lib to emulate brk/sbrk using more
standard allocation methods. By default the heap is 64MB, it should be
enough.

util/led/pmfile

@@ -20,7 +20,8 @@ tool_led = cprogram {
 
 	lib_string,
 	lib_object,
-	
+	lib_sbrk,
+
 	outputs = {"%U%/led"},
 	install = {
 		-- FIXME lib.bin in next line needs removing --- pm bug?