Simplify how to redefine macros in 2014/birken

Now one can do

    make clobber PORT=31337 TIME=50000 alt

or whatever.

2014/birken/Makefile

@@ -139,6 +139,14 @@ ${PROG}: ${PROG}.c
 alt: data ${ALT_TARGET}
 	@${TRUE}
 
+# Variables to control port and timing constant in alt code
+#
+PORT= 1701
+TIME= 500000
+
+${PROG}.alt: ${PROG}.alt.c
+	${CC} ${CFLAGS} -DNCC=${PORT} -DSTARDATE=${TIME} $< -o $@ ${LDFLAGS}
+
 # data files
 #
 data: ${DATA}

2014/birken/README.md

@@ -56,13 +56,13 @@ described by the author. See [configuration](#configuration) below.
 To specify the port to say, 31337, try:
 
 ```sh
-make clobber CDEFINE="-DNCC=31337" alt
+make clobber PORT=31337 alt
 ```
 
-To change the `STARDATE` timing constant, try:
+To change the timing constant, `STARDATE`, try:
 
 ```sh
-make clobber CDEFINE="-DSTARDATE=5000000" alt
+make clobber TIME=5000000 alt
 ```
 
 You can combine them both of course.