2004/01/01 EF
   I added support for cross-compilation of the mingw
   version under Linux.
   make/sethost.prl: now accepts targets mgwcross and
   mgwnative, the former for Linux compilation, the latter
   for compilation on a Windows machine.
   There is now a make/mgwcross and a make/mgwnative;
   sethost.prl copies the appropriate one to make/mgw.
   dbext.h modified to use the HAS_ROUND_FUNCTION define,
   which is defined in include/dbhost.mgw, because there is
   a round function in math.h.
   Removed LACKS_ISNAN from include/dbhost.mgw; again, the
   function is in math.h.