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.
Add RunCMake.CompilerChange test to cover use of -DCMAKE_C_COMPILER=cc
to change the compiler of an existing build tree. Also test for proper
failure with -DCMAKE_C_COMPILER="" and no CC in the environment.