Enable configurable CC and LD.

--- Makefile	2015-03-16 11:50:36.000000000 +0100
+++ Makefile	2015-10-25 23:55:25.205767890 +0100
@@ -1,8 +1,8 @@
 PREFIX:=/usr
 MANPREFIX:=${PREFIX}/share/man
 
-CC = cc
-LD= ${CC}
+CC ?= cc
+LD ?= ${CC}
 RM = rm
 GZ = gzip
 CFLAGS = -Wall -pedantic