I've been trying to build the current master on Arch Linux, where /usr/bin/python is Python 3.
There are issues in gyb, and various utilities used for tests.
I'm not at all experienced with CMake, but have found this snippet while googling:
set(Python_ADDITIONAL_VERSIONS 2.7)
Including this in CMakeLists.txt in LLDB and Swift makes those build.
Replacing #!/usr/bin/env python with #!/usr/bin/env python2.7 in various test utilities then made at least a big bunch of tests pass.
Building Foundation also requires a shebang change in /configure.
I would be happy to put up some pull requests for these changes, but would like some advice wether this is the right approach to make CMake use Python 2.7, or wether there is a better way.