bb612fdafa Set rustv_1_53 in build script (Tobin C. Harding)
Pull request description:
The rust version is supposed to be set by the build script so that users automagically get features matching the toolchain in use. Currently we have a feature in the manifest for `rustv_1_53` instead setting a compiler conditional configuration option in the build script. This causes `cargo +1.41.1 --all-features check` to fail.
## Note
I don't see `rustv_1_46` used anywhere, do we need that still?
ACKs for top commit:
apoelstra:
ACK bb612fdafa
Kixunil:
ACK bb612fdafa
Tree-SHA512: f74195d4ee5a5bc5f209e99d30789df3552cef10aee5ea8b61a5a701b753999c34d04be9fe0321ccee7a8ec14fa5a05e0b454b9dc5f8deddd7b5b8d4f3d7e744