add option to statically link libstdc++

Summary: Ubuntu 12.04 doesn't have 4.8 out of the box, but our packages
currently depend on libstdc++6 >= 4.8. Although GCC's ABI policy
guarantees forward compatibility, it's scary to have to upgrade
libstdc++ system-wide.

Add a build option (off by default) to statically link it. Next step
will be to update the packaging scripts for 12.04.

Closes https://github.com/facebook/hhvm/pull/3232

Reviewed By: @ptarjan

Differential Revision: D1444701