need incremental classpath mechanism

I keep wanting to *add* something to my classpath, and having
to restate the entire classpath, including to the installed source.
Since I'm building on several systems, and Java is installed in
different places on each, my Makefile cannot be correct, or must
be modified at each site. I want a way to say something like

-addtopath mydir:myotherdir

and have it tacked onto the beginning, just like -I is in the C compiler