Support building for w64.
Add hack to have different names for 64 bit Windows DLLs.

* ltmain.sh: Prefix the SO number for W64 with a "6".
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We need to stick to libtool 2.4.2 anyway, thus we take the easy way
and hack libtool instead of adding "-release 6" to the Makefile.  This
also avoids an extra dash before the 6 and thus yields a prettier name
than the one produced by the -release option.  The header files and
def files are supposed to be identical for w32 and w64 thus we don't
need to do any special code for them.

Having a different namespace for 64 bit DLLs is such a good idea that
I hope this or a similar feature will be provided by future libtool
versions.
Changes to support W64.

* configure.ac (have_dosish_system): Set for W64.
* src/assuan-defs.h (SOCKET2HANDLE, HANDLE2SOCKET): Add versions for
W64.
* src/assuan-handler.c (assuan_command_parse_fd) [W64]: Use strtoull
to parse an FD.
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Note that these changes are not complete but merely sufficient to
allow building of GpgEX.