Avoids Clang module conflicts when a NEON/SIMD-gated system header (e.g. Graphene's graphene_simd4f_t backend) is reachable from two different .systemLibrary modules on aarch64 Linux: one pulls in the real _Builtin_intrinsics.arm.acle module, the other re-parses the same header textually, and Clang rejects the resulting conflicting uint32_t/__fsid_t redeclarations. __GI_SCANNER__ is the escape hatch these libraries already ship for exactly this class of consumer (a declaration-only binding generator, same as gobject-introspection's own scanner).
