Add in the “-arch” flag for osx gpp compilers
Merge branch 'master' into arch-flags-for-osx

* master:
  Allow specifying a custom aol.properties file from either a project property or a system define
  Use nar.os and nar.arch instead.
  Add a way to specify the OS rather than only relying upon os.name.  This is helpful, for example, for building android and iOS.