Unified build: Keep track of generated header files

If someone runs a mixed unixmake / unified environment (the unified
build tree would obviously be out of the source tree), the unified
build will pick up on the unixmake crypto/buildinf.h because of
assumptions made around this sort of declaration (found in
crypto/build.info):

    DEPENDS[cversion.o]=buildinf.h

The assumption was that if such a header could be found in the source
tree, that was the one to depend on, otherwise it would assume it
should be in the build tree.

This change makes sure that sort of mix-up won't happen again.

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