There are a few changes in glibc-2.10 that might make your software fail to build with it, please refer to my blog post  if you're not sure what the problem is.
And no I don't usually provide emerge --info with these bugs because they are caused by glibc-2.10!
Created attachment 196367 [details]
It also has O_CREAT problems I believe this one fails with 2.9 as well?
*** Bug 274379 has been marked as a duplicate of this bug. ***
Removed from tree by maintainers.