Defining CONFIG_TOOLS on the basis of $(TOOLS) has the disadvantage
of including it also if e.g. qemu-ga is requested.  The correct
information is available in configure, define it there.

This also has the benefit of not installing the manpages for block layer
tools if the only "tool" being built is the guest agent.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>