Build system changes to allow Android cross compilation

Some minor build changes to allow cross compilation to build Android executables. See [my post on building for Android](http://bluishcoder.co.nz/2013/05/09/building-wasp-lisp-and-mosref-for-android.html) to use.

It basically just allows overriding of the OS/PLATFORM/STRIP to the android SDK equivalents and stops '-lrt' if on Android.