Installation fails on Gentoo/amd64

[Elberet]

I believe 2e10567 is causing the build to fail on my system:

Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

    /usr/bin/ruby21 extconf.rb
  -- tar zxvf msgpack-0.5.7.tar.gz
  -- ./configure --disable-dependency-tracking --disable-shared --with-pic --prefix=/home/rpgforum/.gem/ruby/2.1.0/gems/rbtrace-0.4.4/ext/dst/
  -- make install
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib64
        --with-make-prog
        --without-make-prog
        --srcdir=.
        --curdir
        --ruby=/usr/bin/ruby21
/usr/lib64/ruby/2.1.0/fileutils.rb:1573:in `stat': No such file or directory @ rb_file_s_stat - /home/user/.gem/ruby/2.1.0/gems/rbtrace-0.4.4/ext/dst/lib64/libmsgpackc.a (Errno::ENOENT)
        from /usr/lib64/ruby/2.1.0/fileutils.rb:1573:in `block in fu_each_src_dest'
        from /usr/lib64/ruby/2.1.0/fileutils.rb:1589:in `fu_each_src_dest0'
        from /usr/lib64/ruby/2.1.0/fileutils.rb:1571:in `fu_each_src_dest'
        from /usr/lib64/ruby/2.1.0/fileutils.rb:399:in `cp'
        from extconf.rb:54:in `<main>'

From mkmf.log:

Libraries have been installed in:
/home/user/.gem/ruby/2.1.0/gems/rbtrace-0.4.4/ext/dst/lib