Output make-style dependencies with -M.

src/asm/rgbasm.1

@@ -45,6 +45,11 @@ The
 option disables this behavior.
 .It Fl i Ar path
 Add an include path.
+.It Fl M Ar dependfile
+Print
+.Xr make 1
+dependencies to
+.Ar dependfile .
 .It Fl o Ar outfile
 Write an object file to the given filename.
 .It Fl p Ar pad_value