Add missing $(EXT)
Add missing $(EXT)

If the $(EXT) are added, "make install" can work with MinGW build on
Linux.

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso