configure: Use LLVM shared libraries by default

configure: Use LLVM shared libraries by default

Linking with LLVM static libraries is easily broken by changes to
the llvm-config program or when LLVM adds, removes, or changes library
components.  Keeping up with these changes requires a lot of maintanence
effort to keep the build working on the master and stable branches.

Also, because of issues in the past LLVM static libraries, the release
manager is currently configuring with --with-llvm-shared-libs when
checking the build before release.  Enabling shared libraries by
default would allow the release manager to run ./configure with
no arguments, and be reasonably confident that the build would succeed.

Acked-by: Emil Velikov <emil.l.velikov@gmail.com>