don't know what make you're using, [...]
I'am using the make.exe coming with the DMD 2.061 distro in D/dmd2/windows/bin/ Thought/hoped the GNU spec was applicable, but if it works with GNU make and not Digital Mars make, well..
make -man leads to http://digitalmars.com/ctg/make.html, which says:
Implicit Definition lines .targ_ext.dep_ext : [; rule] [# ... ] Does this give a clue?
