Fixed GEOS & GDAL C function prototypes to explicitly use a subclass of...

Fixed GEOS & GDAL C function prototypes to explicitly use a subclass of `c_char_p` as the return type on routines that return strings.  This prevents crashes on some 64-bit platforms, like FreeBSD.  Thanks to wilsaj for discovering the problem.  Refs #9747.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@11205 bcc190cf-cafb-0310-a4f2-bffc1f526a37