gnu: gxmessage: Upgrade to 2.20.1.

* gnu/packages/gxmessage.scm (gxmessage): Upgrade to 2.20.1.

Merge branch 'master' into core-updates

Conflicts:
	gnu-system.am

gnu: gxmessage: New module

* gnu/packages/gxmessage.scm: New file
* gnu-system.am (GNU_SYSTEM_MODULES): Add gxmessage.scm

Signed-off-by: Ludovic Courtès <ludo@gnu.org>