Move variables from librsvg-features.c to Rust
librsvg-features.c only contains a few variables which must exist as symbols in the final librsvg.so
; they are used to obtain the library's version number.
We should move these variables to c_api.rs to remove the C code from the library.
One small complication is that the macros used, e.g.
const guint librsvg_major_version = LIBRSVG_MAJOR_VERSION;
come from the build system. I'm not sure if cargo-c (see #552 (closed)) provides facilities for this sort of thing, or if we need some build.rs hackery.