Add gcc build-dependencies to SDK
The Proton SDK wants to build its own cross-gcc targeting Windows (mingw-w64), in preference to using the packaged cross-gcc inherited into soldier from Debian (which is older).
If we make sure all its build-dependencies are in the soldier image, then building the Proton SDK won't require installing them separately.
Of the packages listed in https://github.com/ValveSoftware/Proton/blob/experimental_6.3/docker/build-base.Dockerfile.in, we only need to add:
- libisl-dev
- libmpc-dev
- libmpfr-dev
No license-compliance implications: we already ship the corresponding shared libraries, so we already have license compliance obligations for those source packages.
/cc @rbernon