eventmonitor 1.0.3 compile problems

[zerolagtime]

I'm trying to build a Dockerfile to contribute to the application. I'm down to the point where this command comes up: git clone https://github.com/GSA/pdf-filler.git && cd pdf-filler && /bin/bash -l -c "bundle install". Everything is pretty quiet until one of the dependent ruby gems called eventmonitor needs to be built. I've tried installing a newer version of eventmonitor, but somebody has v1.0.3 hard coded. Version 1.2.0.1 compiles just fine.
Even if I ask for v1.0.3 to be installed by hand, it fails with errors like these
em.cpp:574:37: error: 'rb_thread_select' was not declared in this scope
EmSelect (0, NULL, NULL, NULL, &tv);
^
em.cpp: In member function 'int SelectData_t::_Select()':
em.cpp:827:67: error: 'rb_thread_select' was not declared in this scope
return EmSelect (maxsocket+1, &fdreads, &fdwrites, &fderrors, &tv);

em.cpp: In member function 'void EventMachine_t::_RunSelectOnce()':
em.cpp:946:40: error: 'rb_thread_select' was not declared in this scope
EmSelect (0, NULL, NULL, NULL, &tv);

Here is the Dockerfile so far:
FROM ubuntu:16.04
RUN apt-get update -y
RUN apt-get install -y curl pdftk git
RUN gpg --keyserver hkp://keys.gnupg.net --recv-keys 09B6B1796C275462A1703113804BB82D39DC0E3
RUN curl -L https://get.rvm.io | bash -s stable --ruby
#RUN /bin/bash -l -c "gem install eventmachine -v '1.2.0.1'"
RUN /bin/bash -l -c "gem install eventmachine -v '1.0.3'"
RUN /bin/bash -l -c "gem install bundler"
RUN git clone git://github.com/eventmachine/eventmachine.git && cd eventmachine && /bin/bash -l -c "bundle install" && /bin/bash -l -c "bundle exec rake compile"
RUN git clone https://github.com/GSA/pdf-filler.git && cd pdf-filler && /bin/bash -l -c "bundle install"
EXPOSE 4567
CMD["ruby", "pdf-filler/app.rb"]