version in /usr/bin. will need env PATH=/usr/local/bin:$PATH on FreeBSD to detect
required flex 2.5.37. Using CMake cache variables to link GMP (fixes FreeBSD build).
September 9, 2013, before CMake changes. Includes proto_headers.h.
Most or all of __FREEBSD__ changes should, IMHO, be merged back into Linux version.
This should build on Linux with __FREEBSD__ defined.