In commit 8b74ab08 (configure.in: no longer
compile shared libraries by default.), we turned off compilation
of shared libraries by default.  However, by using AC_DISABLE_SHARED,
any executable also linked to uninstalled libtool libraries
(such as libpolylib and libpiplib), would dynamically link
against the uninstalled versions, even after installation.

We now only turn off building libbarvinok as a shared library
by default, introducing a new --enable-shared-barvinok] option
to turn it back on.