No longer depend on cc to build the library 4 unix

[nagisa]

Fixes #61

[nagisa]

A couple thing I still want to do before landing this:

1. Look through the list of targets and see if any of them obviously are MT-unsafe in their currently supported releases.
2. Document this as a platform-specific behaviour for Library::get.

[nagisa]

Linux, Solaris, OpenBSD, Redox are MT-safe.
macOS, Android appear to be MT-safe.

FreeBSD, DragonFlyBSD, NetBSD are MT-unsafe.

[nagisa]

Fuchsia appears to be MT-safe. WxWorks is unknown.

[nagisa]

I think I know the best path forward here:

1. Assume that on platforms that were not confirmed to have MT-safe dlerror, dlsym returning a nullptr is a guaranteed error. This makes it infeasible on those platforms to use features (such as IFUNC) that would otherwise allow dlsym to do this without it being an error;
   • Use a whitelist for this, i.e. set a dlerror-mtsafe feature on platforms where we know it is MT-safe;
2. Make dlerror not producing an error a plain Err not a panic like it is currently.

[nagisa]

Bitrig is MT-unsafe.