Compile With Clang and/or with C++11 Enabled

This adds the ability to compile with clang and/or c++11 enabled.

This is a work in progress toward implementing a non intrusive way to use clang + c++11 enablement.  Apparently on OSX, if you want to use an LV2 plugin (like Ingen) with a Gtkmm GUI that is using c++11, then Ardour + all shared dependencies between plugin gui and Ardour also need to be compiled with c++11 support.  This isn't true on Linux because I can already run the lv2 version of Ingen from within ardour.