During `./waf configure`, allow selecting a subset of tools. For every tool, `wscript` should automatically define `--enable-X` option and `--disable-X` option, where X is the directory name for the tool. The semantics of those options is: * If any `--enable-`* option is used, only those tools specified by `--enable-`* options are selected. * If no `--enable-`* option is used, all tools excepted those specified by `--disable-`* options are selected. * It's an error to specify both `--enable-X` and `--disable-X` for tool X. Dependency detection for deselected tools should be skipped. Unit testing for deselected tools should not be built.