Converted most of top level configure.ac and Makefile.am into CMakeLists.txt.
Added cmake versions of config.h.in, pocl.icd.in, pocl.pc.in.
Most of LLVM & Clang related stuff is in cmake/LLVM.cmake module.

Changes against configure.ac setup:
- remove install-paths.h references (most are unused anyway); replace
  PKG*DIR variables from install-paths.h with defines from config.h
- replace PACKAGE_VERSION with POCL_VERSION
- replace CLFLAGS with KERNEL_CL_FLAGS, and CLANGXX_FLAGS with
  KERNEL_CLANGXX_FLAGS - they seem more fitting and less confusing (with
  LLVM_{CXX,C,CL}FLAGS being values from llvm-config)
- changed TARGET-{LLC,CLANG}-FLAGS into {CELL,TCE}-{LLC,CLANG}-FLAGS
  seems more consistent wrt >1 ocl "targets"
- removed {TARGET,HOST}_SIZEOF_<type> defines, they all seem unused;
  there is some code using SIZEOF_<type> defines though.

Missing stuff:
- HOST_{LLC,CLANG,...}_FLAGS setup is incomplete, especially on !x86, and
  also sanitization of various triplets is missing
- Other platforms than linux need testing, may not work
- TCE & Cell need testing