gnu: rtmidi: Update to 4.0.0.

* gnu/packages/audio.scm (rtmidi): Update to 4.0.0.
[source]: Download tarball from upstream URL rather than the powertab github
fork.
[arguments]: Enable tests.  Remove 'build and 'install phases and add a new
'noconfigure phase to prevent the bootstrap script from invoking configure.
[home-page]: Update homepage to upstream URL.
* gnu/packages/music.scm (powertabeditor)[arguments]: Add phase that substitutes
the rtmidi #include directive to find the header.

Signed-off-by: Mathieu Othacehe <m.othacehe@gmail.com>