Add library functions to the cxx module.
Refactor header to check for OS_POSIX.
Extract executable finding into platform function.
Make OSError constructor public.
Keep track of currently-executing binary.

This allows us to use the same Fabrique binary when re-generating build files.
Regenerate build files with correct executable.

When a Fabrique file changes, we regenerate our build files. Now we can use the same Fabrique binary that we used last time, so the PATH environment variable doesn’t have to hold the binary any more. This resolves #10.