configury: disable building a DL when only --enable-static

If you ```./configure --disable-shared --enable-<foo>=dl```, error out
(because this is an impossible situation -- Libtool can't build a DSO
when we're otherwise configured to only build static libraries).

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
Merge pull request #213 from jsquyres/pr/cant-do-dl-when-static

configury: disable building a DL when only --enable-static