graphics/imv: Update to 3.1.1

This allows us to drop the custom Makefile patch and install files
with the right permissions. Thanks to @eXeC64 for putting out a
new release for this.

While here

• Take it back
• Fix license
• Make sure a2x runs during build instead of install
• Expose options for the new backends
• Make FREEIMAGE support optional
• Use USE_GNOME=librsvg2

Changes: https://github.com/eXeC64/imv/blob/v3.1.1/CHANGELOG