[syslinux] Install syslinux on GPT drive with Windows

[Erik Sandberg]

I'm trying to make a drive that can boot with both UEFI and BIOS. I've
built the drive successfully on my Linux machine, but can't get the drive
to build with my Windows machine. The only step that's different right now,
is that I use `extlinux --install /media/drive` to install with Linux, and
`syslinux.exe --install d:` to install with Windows. In fact, I can prepare
a drive with Windows, try and fail to boot, then stick it in my Linux
machine and run `extlinux --install /media/drive` and then it will boot.
Note that this is a USB flash drive that I've formatted with diskpart as a
gpt, enabled the gpt legacy boot flag, and installed gptmbr.bin to the
first 440 bytes of the disk. The only thing that I can't figure out is why
extlinux --install works but syslinux --install doesn't. Does anyone know
why this might be?