Build the cmake module for our runtime as xtask, part 2

This is done by calling cargo with json output to build the libraries,
collect the .a files, extract the native libraries needed for final
linkage and pass all that to a CMake project that cobbles together the
.a files into a propery cmake target with include paths, etc.

api/sixtyfps-cpp/cmake/lib.cpp

@@ -0,0 +1 @@
+/* Dummy file to make cmake happy */