Prefer COMPUTE over DISPLAY (for AMD OpenCL). This means you can
eg. export COMPUTE and then run "ssh -X" without hassle. Also,
assume :0 if lacking both.
OpenCL: stdlib.h needed for setenv() on some systems.
OpenCL: _BSD_SOURCE needed for setenv() on glibc.
tweaks required for building on FreeBSD 10 (32-bit)
Merge pull request #489 from kholia/FreeBSD10-fixes

tweaks required for building on FreeBSD 10 (32-bit)