Don't delete SRCDIR/zconf.h when building out of tree

Don't delete SRCDIR/zconf.h when building out of tree

This avoids dirtying the source directory when building out of tree

Various build changes to make sure this works:
 - Includes for arch dir specify BUILDDIR before SRCDIR
 - #include <zconf.h> instead of "zconf.h": that way the include order
   takes effect.
 - Insert an #error directive in the original zconf.h that we process out.
   That way we can be sure we are including the right one when building
   out of tree.

Signed-off-by: Daniel Axtens <dja@axtens.net>