Log Message:
lang/gnat-aux: Modify rpath handling for shared project libraries

The GNAT compiler project builder essentially doesn't support DESTDIR
out of the box.  By default, it sets rpath of shared libraries to the
directory to which they are installed.  One may add additional rpaths
through switches, but not remove these default ones.  Also added to
the default rpath are the paths to the ada library and the standard
localbase library.

This modification to the compiler will force the project builder to
recognize the -R switch (gnatlink uses this to disable rpaths), and
it reacts by not putting the library install path into rpath.  The
adalib and ${LOCALBASE}/lib paths will still make up the base rpath
definition of the built shared libraries.

This change was prompted by the rpath troubles of the XML/Ada package.