Make result of DEFINE-MODIFY-MACRO available at compilation time.

Thanks to Cyrus Harmon.

Fixes <http://abcl.org/trac/ticket/368> in support of not needing the
ugliness of
<https://github.com/easye/cffi/commit/03624e609cd48cc065bd15c8dcea715f287b6bbc>.

git-svn-id: http://abcl.org/svn/trunk/abcl@14727 1c010e3e-69d0-11dd-93a8-456734b0d56f