Enable C++11 define when the compiler does.

* Renames HAVE_CXX0X to HAVE_CXX11 to be in line with the offical name.
* Depends on __cplusplus being defined as >= 201103L.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56386 75b3775d-3801-0410-9e12-e2e43121521d
Add a way to use the C++11 virtual specifiers.

The have not been used yet, but they will be added in the future.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56387 75b3775d-3801-0410-9e12-e2e43121521d
Remove an unneeded semicolon.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56388 75b3775d-3801-0410-9e12-e2e43121521d
Remove some unneeded semicolons.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56389 75b3775d-3801-0410-9e12-e2e43121521d