Comment default CC assignment

CC by default is cc, so the assignment was doing nothing, but
it was using non standard syntax, so some system (NetBSD) fail
to compile.

config.mk

@@ -24,5 +24,5 @@ CFLAGS += -g -std=c99 -pedantic -Wall -Wvariadic-macros -Os ${INCS} ${CPPFLAGS}
 LDFLAGS += -g ${LIBS}
 
 # compiler and linker
-CC ?= cc
+# CC = cc