File Source/WebCore/PlatformMac.cmake includes SourcesMac.txt, even though it was removed in commit d0fd15513d412ff586993444fc5a547b114a1eb9.
If you're building on a Mac its better to use XCode at this time. The CMake build for Mac is more experimental since there's no buildbot associated with it.
*** This bug has been marked as a duplicate of bug 192658 ***