[Extensions] Compile extensions as a static library

Since extensions shouldn't depend on other parts of Crosswalk, we can
compile it as a static library. One advantage is that now both
xwalk_runtime and xesh depend on that library, avoiding multiple
compilation of some source files.

NOTE: This PR is built on top of https://github.com/crosswalk-project/crosswalk/pull/1334, that should land before this one.