First point to note is that you’re using a separate project, independent of MRtrix3, which we don’t provide support for. We would generally suggest you install an official release of MRtrix3 before investigating.
Nonetheless, I think in your case the issue is likely to be simply that the dash you used in the EIGEN_CFLAGS="–isystem ..." variable is a long em-dash, rather than the regular hyphen “-” (there are many dashes in Unicode, but only the ASCII hyphen / minus sign would be recognised as such). I suspect you may have copy/pasted that bit from a Word document or similar, which often decides to ‘upgrade’ hyphen to long dashes. – see e.g. this post for details.
To fix this, simply pull up that command again, and edit it to replace the dash with a minus sign that you’ve typed in from the keyboard. Hopefully that’ll be all that’s required here…