Enhance cross-platform build system (#266)

With this new build system, it'll make it easier to compile third-party
libraries statically to multiple platforms.

dockerfiles/linux-x64.Dockerfile

@@ -0,0 +1,8 @@
+FROM dockcross/linux-x64
+
+ARG IMAGE=lherman/cross-linux-x64
+ARG VERSION=latest
+ENV DEFAULT_DOCKCROSS_IMAGE ${IMAGE}:${VERSION}
+
+COPY init.sh /tmp/init.sh
+RUN bash /tmp/init.sh