Make a status message more clear
Fix the search for libOpenCL

- use Pkg-config then fallback to find_library
Fix up cl.hpp + OpenGL checks

- when cl.hpp is not found, use pocl's own cl.hpp, but
  OpenGL's gl.h must be present! (it's #included by cl.hpp)
Fix CL_DISABLE_LONG config.h variable
Revert a few changes to be compatible with autotools

- Use install-paths.h again
- PACKAGE_VERSION
Change pthread device's support of doubles to depend on CL_DISABLE_LONG

Seems better suited than checking SIZEOF_DOUBLE on host. Also remove a bunch
of unused SIZEOF_* variables.