In case of statically linking LLVM into libpocl, resolve LLVM symbols
at build time.
This fixes the segfault that happened when the OpenCL application also
links in OpenGL (e.g. smallptgui), and that OpenGL is implemented with
mesa's LLVMpipe. Mesa exposes the LLVM symbols, and pocl jumped to those,
causing the error.

In standalone mode, this caused double definitions of the LLVM symbols,
so do not link LLVM libs to the llvmopencl library when in API mode, it
works fine without.