Currently, bsb.exe assume that bsc.exe and bsdep.exe are sitting in the same directory as bsb.exe.

And it try to figure out the path of bsc.exe by using Sys.executable_name.
Note this is not nice that Sys.executable_name does not always return an absolute path, if it returns
a relative path(it can in theory change each time of invocation), we have to adjust it to the working dir lib/bs, the worse thing is that if we change the path of bsb.exe, it may impact the path, and ninja will recalculate all deps (the ninja behavior is correct)

A simple solution in the future would be just hard code the absolute path of bsb.exe during configuration, and we add a check if bsb.exe's path has changed during each execution

Also allow people to configure bsc.exe and bsdep.exe seems to be a bad idea
@chenglou