x86_64 build

Hi guys,

To build Xnest on x86_64, I must use the following:
  make CFLAGS=-D__AMD64__
to get GLYPHPADBYTES and GETLEFTBITS_ALIGNMENT set correctly.
Xnvidia and Xvesa both need vm86.h, which doesn't exist
in my glibc.

On my laptop, which also has nVidia graphics, using
1280x1024, it *looks* like the color depth is really low.
1600x1200 is double high, showing only the top half, with
both Xvesa and Xnvidia.  I haven't had time to look at it
much yet, but thought I'd pass along the report.

Nice work on this project.

Thanks,
Dave