[build-users] [build2] Compile fixes

[Ahmed Charles]

I had issues getting build2 to compile with gcc (4.9) or clang (Apple 6.1). Here are the changes I needed to get it to compile with gcc, though it still doesn't compile with clang.

https://github.com/ahmedcharles/build2

I also included a bootstrap script to make it more obvious what people have to do to get a working b executable, at https://github.com/ahmedcharles/build2/tree/bootstrap