libiio's Python bindings use ctypes to load the shared library, which
obviously can't work with a static build. This has recently started
causing eval errors because the package uses
stdenv.hostPlatform.extensions.sharedLibrary, which isn't available when
building a static library.
This patch adds a flag to disable the Python bindings, and automatically
disables them for static builds.
Note that even with this patch, static libiio doesn't build successfully
by default. You have to disable avahi and manually disable libxml2 with
custom CMake flags.
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