Configure et al: treat C defines separately

With some compilers, C macros are defined differently on the command
line than on Unix.  It could be that the flad to define them isn't -D,
it could also be that they need to be grouped together and not be mixed
in with the other compiler flags (that's how it's done on VMS, for
example).

On Unix family platform configurations, we can continue to have macro
definitions mixed in with the rest of the flags, so the changes in
Configurations/*.conf are kept to an absolute minimum.

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