Replaced autocheck exprtk with CPM.

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).