execute_process() does not take ARGS
gcc 6 finally seems to complain about this:
CMake Error at VTK/CMake/vtkCompilerExtras.cmake:47 (if):
if given arguments:
"gcc: error: ARGS: No such file or directory
gcc (GCC) 6.0.0 20160129 (Red Hat 6.0.0-0.7)
Actual problem is not matching on the leading 6. Not sure why the limited regex there.