diff -u a/Makefile b/Makefile
--- a/Makefile	2011-05-14 22:24:57.000000000 -0400
+++ b/Makefile	2016-04-01 14:53:05.000000000 -0400
@@ -19,9 +19,9 @@
 
 #****************************************************************************
 
-CC     := gcc
-CXX    := g++
-LD     := g++
+CC     ?= gcc
+CXX    ?= g++
+LD     ?= g++
 AR     := ar rc
 RANLIB := ranlib
 
Common subdirectories: a/docs and b/docs