Add warning for FORCE_STATIC libraries when using BUILD_SHARED_LIBS.

We cannot built ISL as shared object because we build it with
-fvisibility=hidden; The created shared object would have no accessible symbols.

The reason it is built with -fvisibility=hidden is because opt/clang might load
other libraries that have ISL embedded and whose' symbols would conflict with
Polly's private ISL. This could happend with Draggonegg.

In the future we might instead statically link PollyISL into the Polly shared
object to avoid installing the static library.

Requested-by: Vedran Miletic <vedran@miletic.net>

See-also: llvm.org/PR27306
llvm-svn: 279737