Add a compile time switch to enable pocl debug messages.

Introduces POCL_DEBUG_MESSAGES (and HAVE_CLOCK_GETTIME) config.h variables
into cmake (-DPOCL_DEBUG_MESSAGES=1) and autotools(--enable-debug-messages).
Add debug message macros to pocl_cl.h
Fix includes of pocl_util.c
- pocl_cl.h is included by pocl_util.h already
- pocl_runtime_config.h is required for pocl_get_bool_option()
Change POCL_ABORT_UNIMPLEMENTED macro to take a string argument
(a message to print before aborting).
Add debug messages for clBuildProgram
Add debug messages for clCreateBuffer
Add debug messages for clCreateCommandQueue
Add debug messages for clCreateContext
Add debug messages for clCreateContextFromType
Add abort messages to clCreateFromGLTexture*
Add debug messages for clCreateImage
Add debug messages for clCreateKernel
Add debug messages for clCreateKernelsInProgram
Add debug messages for clCreateProgramWithBinary
Add debug messages for clCreateProgramWithSource
Add debug messages for clCreateSubBuffer
Add debug & abort messages for clCreateSampler
Add abort message for clCreateUserEvent
Add debug messages for clEnqueueBarrier
Add buffer bound and overlap checking functions to pocl_util.c/.h