Add alt code to 1989/ovdluhe + update README.md

The alt code is based on the author's remarks. It allows one to more
easily redefine the macro they refer to (P) in a way that lets them try
it out without having to modify the code.

Updated .gitignore.

Format fixes in README.md.

1989/ovdluhe/Makefile

@@ -115,8 +115,8 @@ OBJ= ${PROG}.o
 DATA=
 TARGET= ${PROG}
 #
-ALT_OBJ=
-ALT_TARGET=
+ALT_OBJ= ${PROG}.alt.o
+ALT_TARGET= ${PROG}.alt
 
 
 #################
@@ -138,6 +138,9 @@ ${PROG}: ${PROG}.c
 alt: data ${ALT_TARGET}
 	@${TRUE}
 
+${PROG}.alt: ${PROG}.alt.c
+	${CC} ${CFLAGS} $< -o $@ ${LIBS}
+
 # data files
 #
 data: ${DATA}