PTHREAD_*_MUTEX_INITIALIZER_NP constants added in #960 are not tested

[glandium]

Because ctest never sets target_endian, no unit tests are generated for the constants I added in #960.
Before doing any release of libc with those constants, ctest should be updated with a version that has https://github.com/alexcrichton/ctest/pull/29 merged.

[glandium]

Testing current libc against the updated ctest: https://travis-ci.org/glandium/libc/builds/364910409

[glandium]

Now that the ctest PR was merged, a PR to update Cargo .lock here will give results more quickly.