Build: Make it possible to have defines assigned to end products as well

This simple fix allows the following construct:

    PROGRAMS=foo
    SOURCE[foo]=foo.c bar.c
    DEFINE[foo]=FOO=1 BAR=0

These will trickle down to the build of object files, so building
foo.o and bar.o will be done with these options: -DFOO=1 -DBAR=0
(exact syntax depending on platform, of course)

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7553)