In net-misc/curl-7.29.0 (and possibly a some older versions as well, I haven't looked there), the "nonblocking" USE flag does not do anything. a) it is not an option in the configure script b) the CHANGES file has this to say: Daniel Stenberg (1 Sep 2012) [...] - configure: remove the --enable/disable-nonblocking options [...] So I am hereby asking to remove this flag from the latest ebuild. Thanks!
Okay I verified and removed it in 7.29.0. I don't know if older versions could have this flag removed, but I'm looking to stabilize 7.29.0 in a month or earlier since it has some critical fixes.