Installation (via gem install) is failing again on Windows 10

ext/rugged/extconf.rb、特に33行目の run_cmake から始め、示されている Ruby 2.6 環境で Windows 10 上で gem install licensee を使って失敗を再現します。ネイティブ拡張のビルドパスを確認し、報告されている pgroup ArgumentError が発生せずにインストールが完了することを確認します。

Seems the pgroup error--which was fixed recently in #791--is back... 😢

Key line below is:

extconf.rb:33:in `spawn': wrong exec option symbol: pgroup (ArgumentError)

gem install licensee
Temporarily enhancing PATH for MSYS/MINGW...
Building native extensions. This could take a while...
ERROR:  Error installing licensee:
        ERROR: Failed to build gem native extension.

    current directory: C:/tools/ruby26/lib/ruby/gems/2.6.0/gems/rugged-0.28.4.1/ext/rugged
C:/tools/ruby26/bin/ruby.exe -I C:/tools/ruby26/lib/ruby/2.6.0 -r ./siteconf20200116-30660-pcywhb.rb extconf.rb
checking for make... yes
checking for cmake... yes
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog
        --without-make-prog
        --srcdir=.
        --curdir
        --ruby=C:/tools/ruby26/bin/$(RUBY_BASE_NAME)
        --with-sha1dc
        --without-sha1dc
        --use-system-libraries
extconf.rb:33:in `spawn': wrong exec option symbol: pgroup (ArgumentError)
        from extconf.rb:33:in `run_cmake'
        from extconf.rb:104:in `block (2 levels) in <main>'
        from extconf.rb:101:in `chdir'
        from extconf.rb:101:in `block in <main>'
        from extconf.rb:98:in `chdir'
        from extconf.rb:98:in `<main>'

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  C:/tools/ruby26/lib/ruby/gems/2.6.0/extensions/x64-mingw32/2.6.0/rugged-0.28.4.1/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in C:/tools/ruby26/lib/ruby/gems/2.6.0/gems/rugged-0.28.4.1 for inspection.
Results logged to C:/tools/ruby26/lib/ruby/gems/2.6.0/extensions/x64-mingw32/2.6.0/rugged-0.28.4.1/gem_make.out