Flatpak support

I had some trouble with the Flatpak version of Steam. Eventually I found a way to make libstrangle work by patching the makefile, and running sudo make install-flatpak. This libstrangle-flatpak.patch assumes that the 21.08 version of runtime org.freedesktop.Platform.Compat.i386 and runtime org.freedesktop.Platform are installed.

This patch works for the one game I use it for but maybe not for others. So proper (official) support would be nice.

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