xsp is not ready for mono-4+'s removal of gmcs. See bug 558064. Possible solution: add code code like has_version '>=dev-lang/mono-4.0.3.20' && export GMCS=mcs somewhere before econf
I'm trying to install dev-util/monodevelop stable, and ran into this bug using GCC 5.3. When I tried masking mono4 in favor of mono2, mono2 does not build using GCC 5.3.
It seems this was moved to www-servers/xsp
*** This bug has been marked as a duplicate of bug 557588 ***