Only announce double/half extensions in DeviceInfo, if its really supported

SIZEOF(__fp16)==2 is not a good test of half support by clang, replace it
by result of half test (_CL_DISABLE_HALF).
Fix issue 113 in autotools

LLVM support for halfs got better in 3.5, so the cmake test now passed,
but pocl still doesn't compile with halfs -> make the test stricter.