configure.ac: Force use of LLVM shared libs with --enable-opencl v2

configure.ac: Force use of LLVM shared libs with --enable-opencl v2

If we build clover with LLVM static libraries, then clover and also each
pipe_*.so driver that is built will contain their own static copy of
LLVM.  The recent automake changes have uncovered a problem where
the pipe_*.so drivers try to use clover's LLVM symbols.  This causes
LLVM's static registry objects to be initialized each time
a pipe_*.so driver is loaded by clover.  Initializing these objects
multiple times is not allowed and leads to assertion failures in the
LLVM code.

We can avoid all these problems by having clover and all the pipe_*.so
drivers link against the same LLVM shared library.

https://bugs.freedesktop.org/show_bug.cgi?id=59334
https://bugs.freedesktop.org/show_bug.cgi?id=59534

v2:
  - Fix shared library detection when LLVM is built with CMake