FreeBSD support in build.sh script.

With these changes it is possible to use build.sh to build EventStore in FreeBSD (possibly in OS X as well, can't check).

To build on FreeBSD you will have to export CC and CXX symbols with path to correct gcc and g++ versions. On FreeBSD default GCC is 4.2.x, which can't build v8, so you have to install newer version from ports, for example gcc46.

Then you can build with a command:

    export CC=/usr/local/bin/gcc46; export CXX=/usr/local/bin/g++46; ./build.sh full