Do not compile php.exe for Appveyor builds

As mentioned in this comment, it should be possible for Appveyor to build the extension DLL without compiling php.exe:

<weltling> Derick, btw. perhaps you could check the phpize based approach https://github.com/krakjoe/apcu/blob/master/appveyor.yml
<weltling> you can spare some time per single job by not building all the php, still need to fetch deps in your case
<weltling> see the build_script: section in the yml file
<weltling> for the tests - also of course php is needed, but in that case it fetches the precompiled version, too
<weltling> so except the extension itself, in most case nothing needs to be compiled
<weltling> with some exts i was helping out, the build time dropped reasonably
<weltling> here it fetches https://github.com/krakjoe/apcu/blob/master/appveyor.yml#L99
<weltling> basically, there it fetches the dev package, generates a job file and runs it, all packed into yml
<weltling> if there are no external deps, it's a way easier and faster approach, with deps might need some more lines

The php-sdk-binary-tools package will come in handy, as we can use that to setup the build environment and download deps.