FindScotch() requires libscotcherrexit
When using an old scotch (compiled without cmake), FindScotch() requires libscotcherrexit, which is an error : scotch provides 2 error handling libraries (libscotcherrexit and libscotcherr), and only one of the two is mandatory. Therefore, a scotch install could provide only libscotcherr, and FindScotch() could fail. Solution proposed : either remove detection of libscotcherrexit in FindScotch(), or implement a mecanism to requires 'libscotcherrexit XOR libscotcherr'