Build portability is complicated by recent zlib-ng updates

[ctsa]

I'd like to use rust-htslib in an application where we can retain a straightforward build process for open-source users. One benchmark for this use case is to ask how hard it is to build the tool for a Centos7 cluster without root.

Prior to the 0.38.0 release, rust-htslib wasn't complicating build portability by the above criteria - centos7 + stock gcc + rustup seemed to be sufficient. The new changes adding zlib-ng support now require cmake v3.12+, which I can't find a way to workaround with feature selection, e.g. default-features = false. Is there a feature switch which would remove the cmake requirement or at least reduce it to cmake 2.8?