emulators/openmsx: Update to 0.15.0

While I'm here:

• Fix build with DOCS option set to off: The files were installed to a dummy location outside of the stagedir as there's no parameter to disable the installation of such files.
  
  Nevertheless those files were recorded for packaging which led to issues during stage-qa. Thus do another approach by just removing those files when the DOCS option is disabled.

• Add "gl" to USES as using USE_GL alone is deprecated.
• Update WWW field to avoid redirects.
• Pet portclippy.

Changelog:

https://github.com/openMSX/openMSX/releases/tag/RELEASE_0_15_0

PR: 229444
Submitted by: Ryo ONODERA <ryo@tetera.org> (initial patch for 0.14.0)
Approved by: johans (maintainer timeout)