add freebsd compiler flags

This addresses https://github.com/atom/node-oniguruma/issues/10 .
Tested on every compiler on FreeBSD 10.1 (x64):
- gcc-4.6
- gcc-4.7
- gcc-4.8
- gcc-4.9
- gcc-5
- clang-3.4.1 (which is the default compiler)
Still needs setting CC and CXX variables.