how to change the include dir synchronously?

[clee01]

// before bcp (normally use)
#include <boost/foo/bar.hpp>

boost::foo::bar bar;

// after bcp (ideally use)
#include <toast/foo/bar.hpp>

toast::foo::bar bar;

• but now i can only use #include <boost/foo/bar.hpp> with toast::foo::bar bar instead