Remove /lib from DYLD_FALLBACK_LIBRARY_PATH

While the manual page for dyld says the default used when
DYLD_FALLBACK_LIBRARY_PATH is not set is
$(HOME)/lib:/usr/local/lib:/lib:/usr/lib, its code actually says
```
sLibraryFallbackPaths[] = { "$HOME/lib", "/usr/local/lib", "/usr/lib", NULL };
```
as far back as the first version of dyld released in OSX 10.4:
https://opensource.apple.com/source/dyld/dyld-43/src/dyld.cpp.auto.html

(and the manual page was wrong back then too
https://opensource.apple.com/source/dyld/dyld-43/doc/man/man1/dyld.1.auto.html)

It is better to avoid putting more paths than necessary in this variable.

Author: glandium

src/cargo/core/compiler/compilation.rs

@@ -187,7 +187,6 @@ impl<'cfg> Compilation<'cfg> {
                 search_path.push(PathBuf::from(home).join("lib"));
             }
             search_path.push(PathBuf::from("/usr/local/lib"));
-            search_path.push(PathBuf::from("/lib"));
             search_path.push(PathBuf::from("/usr/lib"));
         }
         let search_path = join_paths(&search_path, util::dylib_path_envvar())?;