Made geos-apple's file-converter depend on apple2enh's library.

The geos-apple target will build that library if it doesn't exist.  The `apple2enh.lib` rule in the `Makefile.inc` file doesn't conflict with the library rule in `libsrc/Makefile` because they have different `$(TARGET)` directories when both rules exist.

P.S., We _can_ just add a simple dependency, because the use of individual make instances isolates the processed makefiles, and avoids complications.