* automake.in (&variable_dump): Use %var_type properly.

* automake.in (&variable_dump): Use %var_type properly.
(&macro_define): Enforce better overriding rules.
Handle the special case that used to handle...
(&define_pretty_variable): this.
Hence, don't.
The variables you define are owned by Automake.
(&generate_makefile): PRE_INSTALL and co must not be defined *by
the user*.
(&variable_defined): Now independent from the owner.
(&variable_output, &variable_pretty_output): Adjust to %var_type.