Add a flag indicating whether the type string in generic_info needs to be freed, and use constructor function for generic_info (Fixes #186 on Github)

Build error fix for when CL_USE_SHIPPED_EXT=False and CL/cl_ext.h doesn't define cl_device_topology_amd struct.

Added CL_USE_SHIPPED_EXT Switch in setup.py to use src/c_wrapper/clinfo_ext.h instead of CL/cl_ext.h.

Add clinfo_ext.h file taken from https://github.com/Oblomov/clinfo/blob/master/src/ext.h  version 271b88488df434eb68eb2d05996c39b5d78fe50b.

There are two bugs:
1. If src_pitches or dst_pitches is specified, there is an uninitialised
   variable.
2. If ConstBuffer is given a short array and content==0, the internal
   buffer is not zero-filled, causing garbage to be passed on.

The latter probably affects a lot of other functions, but hopefully in a
positive way.

Fixes #105.