after the normal include directories when looking
  for the version of sqlite to use.
- On OSX:
  * Extract additional include and link directories
    from the CFLAGS and LDFLAGS, if the user has
    bothered to specify them we might as wel use them.
  * Add '-Wl,-search_paths_first' to the extra_link_args
    for readline and sqlite. This makes it possible to
    use a static library to override the system provided
    dynamic library.