Fixes to config.h.cmakein to set GMX_QMMM_ORCA.

Installing using ORCA for QM calculations would proceed without
any issue, but the mdrun executable was nonfunctional because
GMX_QMMM_ORCA was never #defined, so the code was not correctly
compiled.

Fixes #1286

Change-Id: I230ded2d3cdbd9c5fc3e865d0b20049effb16766