Check if libraries are present using dlopen

[FnControlOption]

Fixes this error from crystal i:

cannot find -liconv
Linker arguments: -L/usr/local/lib -L/usr/local/Cellar/libevent/2.1.12/lib -levent -liconv
Search path: /usr/lib:/usr/local/lib:/usr/local/lib:/usr/local/Cellar/libevent/2.1.12/lib (Crystal::Loader::LoadError)

[Blacksmoke16]

I'm going to take a guess and say the solution here is to install iconv instead of just not linking it? Because this is probably going to have unintended side effects for non crystal i usages.

EDIT: IIRC it was talked about at least moving this behind a flag, e.g. -Dwithout_iconv, which might be a better idea if this is indeed wanted.

[FnControlOption]

Looking into this further, the problem comes from libiconv.dylib not existing anymore on macOS 11 and up. The solution is to call load_file? (which eventually calls LibC.dlopen) on each path and check if the returned value is nil:

New in macOS Big Sur 11.0.1, the system ships with a built-in dynamic linker cache of all system-provided libraries. As part of this change, copies of dynamic libraries are no longer present on the filesystem. Code that attempts to check for dynamic library presence by looking for a file at a path or enumerating a directory will fail. Instead, check for library presence by attempting to dlopen() the path, which will correctly check for the library in the cache.

https://developer.apple.com/documentation/macos-release-notes/macos-big-sur-11_0_1-release-notes

[straight-shoota]

Thanks for digging into this. That's really weird and unexpected behaviour 🤦

I'm wondering whether to keep the old behaviour on non-darwin systems. But I suppose it really doesn't matter to check for the file's existence beforehand. That's one of the first dlopen will do and quickly errors when the file is missing.

[asterite]

It works, thank you!

[Sija]

Since it turned out to be a generic change, the platform:darwin label should be removed.

[straight-shoota]

Let's leave the darwin tag. This change specifically addresses a bug on darwin.