Set the minimum required version of CMake high enough to avoid the
warning for CMAKE_LEGACY_CYGWIN_WIN32. The warning appears on stderr
and breaks the expected output matching.
Previously CMake silently accepted the empty string and added a bogus
dependency on the current directory. Instead warn about the empty file
name and ignore it. We cannot make this an error because there may be
existing projects that accidentally depend on the old behavior.
Add a RunCMake.include test to cover this case.