* builds/dos/detect.mk: Test not only for `Dos' but for `DOS' also.
* builds/dos/dos-emx.mk, builds/compiler/emx.mk: New files for
EMX gcc compiler.
* builds/dos/detect.mk: Add target `emx'.
* builds/compiler/watcom.mk (LINK_LIBRARY): GNU Make for DOS doesn't
like a trailing semicolon; add a dummy command.