Add support for building against external tinyxml2 - Redmine #1956

libxml2 support was removed in favour of bundled tinyxml2 (3.0.0), but there’s no support for building against a system version of the tinyxml2 library. Please add it.

(from redmine: issue id 1956, created on 2016-05-12 by rathann, closed on 2016-06-22)

• Changesets:
  • Revision 013ca6eb by Mark Abraham on 2016-06-07T16:31:50Z:

Re-add support for linking against external TinyXML-2

I always intended this support, to permit convenient packaging of
GROMACS by distributions, but it got lost from gerrit while rebasing
from patch set 4 to 5 of I6153136.

Fixes #1956

Change-Id: Ie76dc9e8c6116814439d813d5a9555c5bfb7bfc5