Remove cargo::rustc-check-cfg from build.rs

[jayvdb]

Fixes #67

[samanpa]

Will be great to get this merged or at least declare a MSRV

[jayvdb]

Adding the following to your Cargo.toml will workaround this problem

[patch.crates-io]
# Workaround for https://github.com/zowens/crc32c/issues/67
crc32c = { git = "https://github.com/jayvdb/crc32c", branch = "crc-i67" }