Add gnu89 std version (#33)

This enables compatibility with clang, which, unlike gcc, errors
(instead of warns) on depreciated C89 constructs like implicit int
unless c89/gnu89 is specifically selected.

Makefile

@@ -1,4 +1,4 @@
-CFLAGS = -w -O
+CFLAGS = -std=gnu89 -w -O
 
 CLEANFILES =				\
 	lisp				\