[cmake] Make it possible to override CL_DISABLE_HALF.
[cmake] Make it possible to override CL_DISABLE_HALF.

I suspect the check is currently wrong. It seems to be checking whether C
allows half precision types in arguments and returns. On Mips, this is not
currently allowed. However, I suspect OpenCL allows this regardless of
target.

This patch removes the unconventional caching mechanism for
CL_DISABLE_HALF in favour of cmake's usual method which allows the user
to specify it with -DCL_DISABLE_HALF=ON/OFF. This allows me to investigate
whether the check is correct or not.

Oblomov/pocl,pavanky/pocl,jrprice/pocl,vkorhonen/pocl,jrprice/pocl,franz/pocl,pavanky/pocl,pavanky/pocl,jrprice/pocl,dsandersimgtec/pocl,0charleschen0/pocl,pocl/pocl,dsandersimgtec/pocl,franz/pocl,Oblomov/pocl,vkorhonen/pocl,rjodin/pocl,franz/pocl,pavanky/pocl,franz/pocl,Oblomov/pocl,rjodin/pocl,Oblomov/pocl,pocl/pocl,jrprice/pocl,franz/pocl,pocl/pocl,0charleschen0/pocl,dsandersimgtec/pocl,vkorhonen/pocl,pocl/pocl,rjodin/pocl,0charleschen0/pocl,dsandersimgtec/pocl,pocl/pocl,vkorhonen/pocl,0charleschen0/pocl,Oblomov/pocl,rjodin/pocl