configure: change JSON test to make it work on Windows
configure: change JSON test to make it work on Windows

Seems g++ has trouble with headers specified by absolute path...

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3