The macro __CYGWIN64__ should be replaced with __CYGWIN__ in src/base/mutex.h.

Cygwin documents that there is no __CYGWIN64__ macro: Cygwin FAQ

This was causing compilation issues for me due to include order problems, I think, when I was building https://github.com/mmp/pbrt-v3 (which includes glog).