Add ability to set the suffix for libraries

[nirbheek]

For shared modules on macOS, the library suffix is often .so, but cargo will always create .dylib for cdylibs.

Is there a way for cargo-c to tell cargo what suffix to use for a shared library?

[lu-zero]

I need to check if cargo has a mean to override that. That I recall is hardwired, but had been long since I looked that part.

[amyspark]

See also rust-lang/cargo#1706 (comment)