The logic to figure out the combinations of --prefix and --openssldir
has stayed in Configure so far, with Unix paths as defaults.

However, since we're making Configure increasingly platform agnostic,
these defaults need to change and adapt to the platform, along with
the logic to combine them.

The easiest to provide for this is to move the logic and the defaults
away from Configure and into the build files.

This also means that the definition of the macros ENGINESDIR and
OPENSSLDIR move away from include/openssl/opensslconf.h and into the
build files.

Makefile.in is adapted accordingly.
Reviewed-by: NRich Salz <rsalz@openssl.org>