Don't let 'generate' target depend on generated files, act directly instead

One of the 'generate' targets depended on $(SRCDIR)/apps/progs.h,
which depended on...  nothing.  This meant it never got regenerated
once it existed, regardless of need.  Of course, we could have it
depend on all the files checked to generate it, but they also depend
on progs.h, so we'd end up getting cricular dependencies, which makes
make unhappy.

Furthermore, and this applies for the other generated files, having
them as targets means that they may be regenerated on the fly in some
cases, and since they get written to the source tree, this isn't such
a good idea if that tree is read-only (which is a possible situation
in an out-of-tree build).

So, we move all the actions to the 'generate' targets themselves, thus
making sure they get regenerated in a controlled manner and regardless
of dependencies.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>