For GP Linux forks which put spidermonkey in a versioned directory like /usr/lib/xulrunner-1.2.3 and also do *not* add it to /etc/ld.so.conf.d, we need to add it to LD_LIBRARY_PATH dynamically.

There are two approaches to dealing with this; compile time and run time.  

* compile time: (for ELF binaries) detect where libmozjs.so lives and add an rpath to it. 
* runtime: Run pkg-config

While compile time would be nice, unfortunately we can't do it for gnome-shell because we're a plugin for mutter.  So what we should do is:

* Detect this situation in configure
* Use it to generate a wrapper script /usr/libexec/gnome-shell/mozjs-wrapper
* Use the wrapper at both compile time (for introspection dumping) and in the installed /usr/bin/gnome-shell script.