Don't generate the headers by default in the source directory, put them
into a sub-directory in OUT_DIR instead and convey that location via
links to the C++ test driver.
Make install would still trigger the generated_headers_target CMake
custom command - due to the use of add_dependencies - and that would end
up running cbindgen. That in turn breaks "make install" when it's run in
an environment that doesn't have cargo in PATH (for example when using
"sudo make install").
This patch folds the generation of the C++ header files into the build
of the sixtyfps-cpp crate - via build.rs. By default the headers are
placed in api/sixtyfps-cpp/generated_include but the CMake build
redirects that into the build directory.
Note: Due to the way that corrosion works, cargo is still run when
running "make install", but it's path is absolute and there should not
be any reliance on the PATH environment variable.
