dev-lang/ruby-1.9.1 doesn't install version.h, but this file is needed by some packages. version.h includes revision.h, so revision.h also should be installed.
Created attachment 182096 [details, diff] ruby-1.9.1.ebuild.patch Workaround.
version.h is now deprecated to my understanding: http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/12069 Extensions that don't consider the deprecation might most likely include other 1.9 incompatible code. Better it breaks at compile time than getting weird errors at runtime. Feel free to reopen if you find contrary upstream statements that are more recent than the one I linked.