Improve fix on 1991/dds - add alt code

The dds.c now does return system(q-6); and the dds.alt.c does the return
system("make a");. This way those who have gcc (more people than not
most likely) they can use it like originally designed. Those with clang
can use the alt target in almost the same way only instead of a.out use
just a.

Updated the README>md to explain everything.

Updated the .gitignore with some globs that should have been there all
along ('a' and 'dds.alt' being the ones that are new).

1991/dds/Makefile

@@ -114,8 +114,8 @@ OBJ= ${PROG}.o
 DATA=
 TARGET= ${PROG}
 #
-ALT_OBJ=
-ALT_TARGET=
+ALT_OBJ= ${PROG}.alt.o
+ALT_TARGET= ${PROG}.alt
 
 
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