Rename scripts/ to build-aux/

* scripts/: Rename to build-aux/
* Makefile.am: Adjust accordingly.
* configure.ac (AC_CONFIG_AUX_DIR): Change to build-aux.
Turn autogen.sh into a generic script.

* autogen.sh: Revamp.
* autogen.rc: New.
* Makefile.am (EXTRA_DIST): Add autogen.rc.
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Signed-off-by: Werner Koch <wk@gnupg.org>