[plt-scheme] back to --disable-shared

In CVS for the exp-tagged code, the default configure mode for Unix is
back to --disable-shared, thus ending a brief experiment with
--enable-shared as the default.

I like the simplicity and flexibility of a single binary, and our
experiment didn't convince me that shared libraries offer a clear
advantage for PLT Scheme, so that's why I changed the default back.

Unix software is often distributed as shared libraries, however ---
especially when source is available, and especially for Linux. People
who know more about distributing software have decided that shared
libraries are usually better, and maybe I'm overlooking some advantage
for PLT Scheme. So, when you're configuring your own build, consider
specifying --enable-shared.

Of course, we still use DLLs for Windows and frameworks for Mac OS X,
because shell scripts make poor application launchers on those
platforms. (In other words, DLLs and frameworks offer a clear advantage
under Windows and Mac OS X, even though they also create plenty of
headaches.)

Matthew