When compiling programs that use the BGL, be sure to compile with optimization.
Why would it be important to enable optimization when compiling programs that use the library? How the heck are you supposed to debug code that uses it? I can understand turning on opt for speed, but the way it's written makes me think they need it for correctness, which is totally crazy.
I guess it's not for correctness, as it is for practical usability. When every character of source code translates into 200 lines of code with 20 nested function calls, you either enable inlining and monomorphization, or end up with hello world that takes 20 seconds to execute and leaves behind 200MB object files