Boost no longer builds starting with 4.0.22

One of the changes in 4.0.22 was the removal of the `em++` and `emcc` script files (with no extension) in the emsdk distributions for Windows.

Unfortunately, boost seems to use at least one of these files for detection when you use b2.exe to build with toolset=emscripten.

So starting with 4.0.22 our boost builds began to fail unless we manually created an empty `emcc` and `em++` file in the `emsdk\upstream\emscripten` folder. I suspect we only needed emcc but we created them both just in case because it's really hard to tell what boost is doing when it builds.

I will report this to boost as well, although in the meantime the fix seems to be creating the missing file stubs before building.