Do not enforce compilers

--- jfbuild/Makefile.orig	2014-11-13 08:43:00.382215199 +0100
+++ jfbuild/Makefile	2014-11-13 08:43:23.230777638 +0100
@@ -55,8 +55,8 @@
   debug=-ggdb -O0 -DDEBUGGINGAIDS -DNOSDLPARACHUTE
 endif
 
-CC=gcc
-CXX=g++
+CC?=gcc
+CXX?=g++
 AS=nasm
 RC=windres
 AR=ar
