configure: Change optimisation flags
configure: Change optimisation flags

For compilation in debug mode, use -O0 rather than -O1.
For linking, use -O0 in debug mode and -O1 in release mode.
Relates to discussion in #1143.
Closes #1072.

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