During the "configure" stage the b2 script tries to stat all directories that are present on system.
This is huge problem when there is some hang nfs mount which is not possible to remove you can't proceed with emerging the package.
Steps to reproduce:
mount nfs mount to /tmp/bla and unplug the cable
emerge -1v boost --nodeps
see it stuck on b2 call, which is not something we really want.
Could not try on testing version sadly but I guess the thing will be still the same.
Basically there is nothing to do with boost or boost-build here.
It's more about NFS itself. How do you mount it, i mean - which options do you use? If you do not limit timeouts and or retries you can stuck with such behaviour even when doing 'ls /path/to/broken/nfs/mount'
So, in my opinion - it's CANTFIX. Sure, it's a problem. But not boost's, definitely