Was not used anymore and is replaced by gltf format (in ParaView).

Submodule commit b0bc24cfdda39b30145b1f6c2c84b801cd131fef was not available anymore in the master branch of ArashPartow/exprtk.
The branch was maybe rebased? Used commit c7c219480d9678eec7383a4a99030683c4a84d91 instead (same commit date).

Closes #3100.

The tool is now included before the benchmark tests are processed.
This should fix an issue reported by @TobiasMeisel where a test
which uses xdmfdiff could not be added to CMake.

Makes hdf5 and libxml2 dependencies optional.

Reverted custom naming for xdmf libs.
Added Iconv as a dependency (dependency of libxml2).

Before this setting when there was a newer system installed python
than the one installed in an active virtualenv the system one was
picked up by CMake thus breaking stuff which depends on the active
virtualenv, such as workflow engine (e.g. Parsl) driven tests.