Make apps/progs.pl not look at apps/progs.c

Make apps/progs.pl not look at apps/progs.c

apps/progs.pl will have apps/progs.c as output, and on some systems,
the output file of a program is locked against reading.
Unfortunately, apps/progs.c is also part of the sources that make up
apps/openssl, so it's necessary to mark that file in a way that makes
progs.pl skip over it.

Fortunately, this is easily done with a special attribute in
apps/build.info and a simple adaptation of apps/progs.pl.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15332)