More Win32 build fixes

[malxau]

These are less significant than the last round. Four things:

1. /YX means "automatic precompiled header", where the first compiler instance compiles headers and stores them in a .pch file, and later compiler instances consume from that file. This scheme assumes serial compilation (only one compiler instance is active at a time), and has been removed from newer compilers which generate a warning instead. This change removes /YX and the /Fp directive that named the precompiled header.
2. Remove more import libraries from WinElvis.exe. This is really something I missed somehow in my last round of changes, so it's finishing what I started from last time.
3. Remove /machine:i386 from linking. The original IDEs inserted this into Makefiles so that you could have a different project with different settings for i386, MIPS, PowerPC and Alpha. The linker will default to a native architecture for the platform it supports, meaning that Visual Studio ships with a linker that generates i386 code, and a linker that generates x64 code. This change just removes the explicit indication and lets the toolchain target the architecture that it supports. Note that this is a linker flag, so the objects have already been compiled, and the linker expects objects in the same architecture that it's linking against, so this flag is insufficient to do anything like cross-compilation. After this change, it's possible to compile as 64 bit, which compiles successfully and runs but there's more scrubbing required to eliminate truncation conditions and ensure it's really correct.
4. One of the makefiles used a variable before defining it, which is actually legal in NMAKE since it originally only performed delayed variable expansion. It doesn't seem compatible with much else though, and there doesn't seem to be a good reason to keep it.