improve configure error message "ERROR: User requested feature nptl"

Running `./configure` on Ubuntu 14.10 amd64 with Linux 3.19.1 causes the error 

    ERROR: User requested feature nptl
           configure was not able to find it.
           Install glibc and linux kernel headers.

Both linux kernel headers and `libglib2.0-dev` are installed in my case, so the error message definitely misses a point and is at least confusing and should either omit the hint if the recommended dependencies are already installed or - better - give one that fixes the issue.

experienced with git commit d598911b6f5e7bf7bafb63b8e1d074729e94aca7