emerging mono-debugger-2.6 with sys-devel/binutils-2.18-r3 installed does not succeed because of missing symbols in binutils module. The mono-debugger-2.6 ebuild should require a version of sys-devel/binutils greater than 2.18-r3. Having sys-devel/binutils-2.20.1 installed permits mono-debugger-2.6 to emerge successfully. Reproducible: Always
Please provide build.log, also check if you are suffering bug 279563 (I suspect this could be a dupe)