Add build prerequisite steps for openSUSE.

Confirmed proper packages by using Dockefile to build from base opensuse.

```Dockerfile
FROM boombatower/opensuse

RUN zypper addrepo --no-gpgcheck \
      http://download.opensuse.org/repositories/devel:languages:python3/openSUSE_13.1/devel:languages:python3.repo && \
    zypper addrepo --no-gpgcheck \
      http://packman.inode.at/suse/openSUSE_13.1/ packman && \
    zypper refresh

RUN zypper --non-interactive install --no-recommends \
  cmake \
  fontconfig-devel \
  ftgl-devel \
  gcc-c++ \
  glew-devel \
  libfreetype6 \
  libogg-devel \
  libopus-devel \
  libSDL2-devel \
  libSDL2_image-devel \
  opusfile-devel \
  opus-tools \
  python3-devel \
  python3-numpy \
  python3-Pillow

ADD . /srv/openage
WORKDIR /srv/openage

RUN ./configure --mode=release --prefix=/usr && \
    make
```