OBJ and EXE and cross-compilation (Was: AC_OBJEXT again)

[Akim Demaille]

Maybe we should just make the difference between cross-compilation or
not?  Here is what I propose.


1. AC_INIT uses uname to compute CYGWIN32, MINGW32 and EMXOS2,
   hence OBJEXT and EXEEXT.

2. AC_LANG_COMPILER if cross-compiling runs the old Cygwin32 etc.
   tests which involve a compilation (__CYGWIN32__).  It changes OBJEXT
   and EXEEXT if needed.

I don't think we need the test we used to perform (compiling and
checking for the names the compiler leaves around).  But if we want to
run this test, then only when not cross-compiling.

There are other things which might be considered.  After all, when
cross-compiling, how about checking $host to compute CYGWIN32 etc?
This would relieve us from having to compile something.  Sure it is
not `clean', but anyway we are already using tables for this case.
There's no way to be sure of the extensions used on the host platform,
since we'd need to run something on it.