For developing your plugin, there is no need to use the latest and 
greatest VM source or VMMaker. You may find it easier to just get a 
stable combination of slightly out-of-date Squeak and source code.

For stable source, look at www.squeakvm.org. For example, if you are 
building something for unix/linux, you can use 
http://www.squeakvm.org/unix/release/Squeak-3.7-7.src.tar.gz This 
contains the platform sources, including sqVirtualMachine.h.

Then just use a stable Squeak 3.7 full image, which will include an 
out-of-date but perfectly servicable VMMaker. The combination should 
work fine for building plugins. And you *should* build a complete VM, 
not just your plugin. It will only take a few minutes extra, and it 
makes it possible to use all the existing configure and make 
procedures. Once you've done this, you can use VMMaker to regenerate 
your plugin as you develop it, and the make procedure will rebuild 
just your plugin as you would expect.

The only caution I would add is to be aware that the up-to-date 
sources are intended to support both 64 bit and 32 bit images, so 
everything that defaulted to type "int" in the older sources is now 
defined as type "sqInt", which might be either 32 bits or 64 bits. 
Just don't assume that anything defaults to an int, and read 
http://www.squeakvm.org/squeak64/ if you want some more background. 
But don't lose any sleep over the issue, it's fairly easy to update 
your plugin for 64 bit later on if you don't feel like worrying about 
it right now.

Dave