Configure: Make it possible to generate mandatory header files

'DEPEND[]=file.h' becomes a special way to say that 'file.h' must be
generated before anything else is built.  It's likely that a number
of source files depend on these header files, this provides a simple
way to make sure they are always generated even it the dependency data
hasn't been added to the build file yet.

Reviewed-by: Rich Salz <rsalz@openssl.org>