Option to specify alternate CMakeLists filename

• Adds cmake --project-file option to specify an alternate filename for CMakeLists files. This determines the top-level file processed when CMake is configured, and the file processed by add_subdirectory(). By default, this is CMakeLists.txt. If set to anything else, CMakeLists.txt will be used as a fallback if the given file cannot be found within a project subdirectory. The use of alternate project file names is intended for temporary use by developers during an incremental transition and not for publication of a final product. CMake will always emit a warning when the project file is anything other than CMakeLists.txt.