* Makefile.in: Revert most of Arnaud's last change. Prefer anything
over error-prone Make abuse. Remove temporary debug code.
(gnatlib-zcx): New target.
(gnattools): Split into gnattools-native, gnattools-cross, to fix
previous bug; select which on the basis of default_gnattools_target.
(ada.all.cross): Merged with gnattools-cross target.
* configure.ac: Set default_gnattools_target.
* configure: Regenerate.