[RFC] A few fixes for some build-related issues.

This should fix an issue discovered in #1370, as well as provide more control if someone needs to select a particular iconv library.  The root of the problem stems from the fact that CMake treats `/sw` and `/opt/local` almost as extensions to the system paths by appending them after the normal system paths, but you quickly run into problems if a dependency is found there.  Since other dependencies offered by the system may also be available in `/opt/local`, you can run into version issues--which was the root cause of the problem discussed in #1370.  So this PR moves them up in priority.

This also fixes an improper use of `LibIntl_INCLUDE_DIR`--it should be `LibIntl_INCLUDE_DIRS`, though it's not a serious issue.
Is there a difference to `if(APPLE)` (used [previously](https://github.com/neovim/neovim/pull/1437/files#diff-af3b638bc2a3e6c650974192a53c7291L36))?