Build on FreeBSD

[aft90]

Hi! Thanks for creating this library. I found myself needing to compile it on FreeBSD (with clang and BSD make which are the default on that platform) so I thought I'd share the changes I had to make:

• added makefiles for the main library (static & shared), test and examples
• added stdio.h to Par.cpp for snprintf
• remove the -Wlogical-op, -Wnoexcept and Wstrict-null-sentinel flags because clang 10 on FreeBSD doesn't recognise them
• turn off some more warnings in System.cpp caused by clang with boost
• added checks for the __FreeBSD__ compiler macro where appropriate