gnu: icecat: Unbundle libevent, libogg, and libvorbis.

gnu: icecat: Unbundle libevent, libogg, and libvorbis.

* gnu/packages/patches/icecat-use-system-media-libs.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/patches/icecat-avoid-bundled-libraries.patch: Add
another hunk to enable removal of libevent.
* gnu/packages/gnuzilla.scm (icecat)[source]: Add patch.  In the snippet, add
libevent, libogg, libvorbis, and libtremor to the list of bundled libraries to
remove.  Add a comment regarding theora.  Remove comments regarding unbundling
cairo, which is no longer supported.
[inputs]: Add libevent, libogg, and libvorbis.
[arguments]: Add --with-system-{libevent,ogg,vorbis} to configure flags.
Add custom bootstrap phase.  Add comments.