[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.
Actually, there is.  `APPLE` is defined if the host you are running on is an Apple operating system.  However, that has nothing to do with the target system.  `CMAKE_SYSTEM_NAME` will be `Darwin` if the target you are building for is Mac OS X.  Think of a cross-compiling situation.  If I end up using MingW to build a Windows executable, we wouldn't want this to trigger.  Hope that makes sense.