Maintainer mode

[Ben Elliston]

After thinking about it a bit, I am ready to propose that the opcodes
and sim Makefile.am's be modified so that they no longer rely on
`--enable-cgen-maint', but instead use the @MAINTAINER_MODE@
substitutions provided by AM_MAINTAINER_MODE to determine whether or
not to include rules for automatically regenerating files.

The advantages here are obvious: you need not remember an extra option
to `configure' and it works more obviously.

Any thoughts before I put it to the gdb and binutils mailing lists?
Hopefully this is a step in the right direction of eliminating the
generated files from the tree altogether.

Ben