Add -MP option for generating phony targets for all dependencies

This patch adds an option that makes SWIG generates phony make targets for all dependencies, e.g.:

<pre>
module.i : \
  header1.h \
  header2.h

header1.h:

header2.h:
</pre>

This is to prevent make from complaining if header files have been deleted before the dependency file has been updated. A similar option exists for gcc, for the same reason.