dist-docs: Make GNU make aware of sub-make.

dist-docs: Make GNU make aware of sub-make.

When GNU make sees that a command to be executed contains the string
$(MAKE), it makes the jobserver that limits parallelism available to
the command.  Otherwise, any sub-make that executes sees that
parallelism is enabled but does not have access to the jobserver, so
it prints a warning and turns off parallel job execution.

This also makes the dist-docs process run the same "make" that is
executed at the top level, in case that's different from the default
"make" found in $PATH.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Russell Bryant <russell@ovn.org>