Fix big in error message spotted by Louie.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@23144 b5f69653-2bf5-0310-8201-b0dafb8e1405
Give a clear error if you've used symlinks to have two copies of the same project,
since otherwise very obscure compiler errors can result.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@23145 b5f69653-2bf5-0310-8201-b0dafb8e1405
For #2628, prefer Xerces 3 as a Chaste dependency.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@23154 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2628 - update ubuntu.py to link against installed version of Xerces.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@23157 b5f69653-2bf5-0310-8201-b0dafb8e1405