Added support for XULRunner/GRE 1.9:

Added support for XULRunner/GRE 1.9:
- Use XPCOM and GtkMozEmbed glue code
  - Do not set rpath in executable
- Use <nsStringAPI.h> not <nsString.h> since the internal string API is not
  exported
  - Do not define MOZILLA_INTERNAL_API macro
- Do not use nsACString::{Begin,End}Reading since they are not exported (bug?)
- Do not add per-module directories to include search path, since they no
  longer exist
- Add NSPR header directory to include search path, since
  mozilla-gtkmozembed-embedding.pc doesn't do that (bug)
- Move use of nsPresContext to a separate source file since its header still
  requires MOZILLA_INTERNAL_API to be defined (bug?)