This commit is to add an extra option to enable verbose mode (V=1) in the generated `Makefile` at runtime of the Ruby to print compiler command lines by the commands below when building native extensions. It's possible to enable the verbose mode by setting the environment variable `MAKEFLAGS="V=1"`[1] implemented in GNU make. However, I wanted to make a consistent user-interface not depending on the specific make's implementation. ``` $ ruby /path/to/extconf.rb -- --with-verbose ``` You can also add the extra option via rake-compiler gem. ``` $ rake compiler -- --with-verbose ``` If the extra option is not given, the value of the `RbConfig::CONFIG["MKMF_VERBOSE"]` enabled by the configure option below is used. ``` $ ./configure --enable-mkmf-verbose ``` For the unit tests, updated the following files. * The `test/mkmf/test_configuration.rb` was created to test the cases with the `configuration` method and this implementation. * Updated the `TestMkmf#assert_separately` to set the extra arguments in `test/mkmf/base.rb`. Updated tests using the `assert_separately`. * Added tests for `MakeMakefile#with_config` in the `test/mkmf/test_config.rb`. [1] https://www.gnu.org/software/make/manual/html_node/Variables_002fRecursion.html Fixes [Bug #19695] Co-authored-by: Nobuyoshi Nakada <[email protected]>