Requires cl_khr_fp16

[nsakabe-fixstars]

I tried on Ubuntu16.04/GTX 1050Ti. ClPy required cl_khr_fp_16 extension.
If this is intended, requirement should be specified in README.md.
Otherwise, some operations that includes half-arguments or return values have to be disabled on build-time.

clpy/core/include/clpy/carray.clh:2:26: warning: cannot enable cl_khr_fp_16 extension
 on this platform - ignoring.
#pragma OPENCL EXTENSION cl_khr_fp16: enable
                         ^
In file included from <kernel>:2:
In file included from clpy/lib/python3.6/site-packages/clpy-2.1.0rc1-py3.6-linux-x86_64.egg/clpy/core/include/clpy/carray.clh:535:
clpy/lib/python3.6/site-packages/clpy-2.1.0rc1-py3.6-linux-x86_64.egg/clpy/core/include/clpy/fp16.clh:15:13: error: declaring function return value of type
 'half' is not allowed; did you forget * ?
static half convert_float_to_half(float f) {

.... (omitted)

[vorj]

#264 is more definite issue (and worth to tackle I think), so this issue should be closed.