Qt5 path in CMakeLists.txt

[Yuki Koyama]

When qt5 is installed via homebrew, the following code would be helpful:

if(APPLE AND EXISTS /usr/local/opt/qt5)
	list(APPEND CMAKE_PREFIX_PATH "/usr/local/opt/qt5")
endif()

Ref: Homebrew/homebrew-core#8392

[Yuki Koyama]

The currently used default path seems /usr/local/opt/qt, not /usr/local/opt/qt5.