Win: Require CMake 3.0+, and look for Qt 5.3 in default location.

Win: Require CMake 3.0+, and look for Qt 5.3 in default location.

CMake 3.0+ has the ability to "find_package(Qt5".  Thus, we will require at
least this version for Windows.

CMake still isn't smart enough to go looking for Qt 5.3, and another
modification is to look to see whether it's installed (on Windows) in the
default location.  If so, set Qt5_DIR to point to it.