cmake: Download Boost if inadequate local version

Building SDL2 from externals is incompatible with Conan's version of
libiconv, a requirement of Conan's Boost package. Solution is to use the
same Boost package in use by the linux-fresh container. This tells CMake
to download boost_1_75_0.tar.xz from yuzu-emu/ext-linux-bin at CMake's
configuration step, much the same way Qt and FFmpeg are downloaded for
Windows.

Also makes DownloadExternals.cmake cross-platform. Although the CMake
code is not entirely specific to Linux, only Linux has Boost libraries
available at ext-linux-bin, whereas there is no equivalent Boost package
for Windows at ext-windows-bin. caveat emptor