link against single libLLVM shared library when available

link against single libLLVM shared library when available

The llvm libraries are linked statically by default.
Linking them into libpet.so may not work reliably and
some linkers will complain about this.
While it is possible to compile shared llvm libraries using
BUILD_SHARED_LIBS, http://llvm.org/docs/CMake.html recommends
against this, proposing to build a single shared library
using LLVM_BUILD_LLVM_DYLIB instead.
Link against this single shared library when it is available.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>