Created attachment 441134 [details] build.log [ebuild U ] dev-ruby/ruby-augeas-0.5.0-r3 [0.5.0-r2] USE="doc {-test}" RUBY_TARGETS="ruby20 ruby21%* (-ruby22) (-ruby23) (-ruby19%)"
Created attachment 441136 [details] emerge --info
*** Bug 589174 has been marked as a duplicate of this bug. ***
This error means that ruby can't compile C code on your system. Have you made any changes to your C environment recently? Does it still work for other applications? One gotcha here is that ruby itself stores some information on the compiler and flags used to compile ruby and it re-uses that in this case. In this case you can fix things by reinstalling ruby itself first.
C complier works fine for other things, will reinstall and get back.