Thank you for the comment. Here are the versions (which I forgot include at the first post)
rustc 1.14.0 (e8a012324 2016-12-16)
cc (GCC) 5.0.0 20150202 (experimental)
Copyright (C) 2015 Free Software Foundation, Inc.
After I looked at the version of ld, I've realized that it's from Apple's XCode-4.0 (older version) and promoted it to the version of XCode-4.2.1. Now it's:
configured to support archs: armv6 armv7 armv7s i386 x86_64
LTO support using: llvm version 3.0svn, from Apple Clang 3.0 (build 209.6)
and after that change, I have rebuilt the hello app and it did start to work! That's fine. But geckodriver still didn't get built after I tried with this change; the same error popped up.
I 've taken a look at the bugfix in rust-1.2. Actually I had saw that post the other day as I searched for a solution to my geckodriver build problem. But I don't know how to apply the workaround they talk about: 'LDFLAGS="-Wl,-no_compact_unwind" ./mach build' ; there, mach seems to be a command but it's not present on my system.
I agree that I should not need to compile Rust to be able to build the geckodriver; especially when I consider that the only problem is a missing dynamic library or rust's being unable to link them. But I still I have to do something to solve it as I basically use Mac's only.
I slightly disagree that only few people are having this issue. Yep, the number of people having this issue is low but that's because the number of people using both BSD Unix and Rust applications for development is low.