Export Target fixes for better transitive properties

[emminizer]

Minor improvements to your PR openscenegraph#63

• Include directory now set on library target for INSTALL_INTERFACE. This makes it so that when linking to the target, you get the install directory added transitively correctly.
• Now using find_dependency in your new .in file, to search for Qt5 and OSG if they don't already exist. Otherwise targets might not be found properly unless the parent project finds them first with the right components.
• Fixed transitivity of link libraries and include directories for OSG. The _OSGQOPENGL_ALREADY_DEFINED block prevents an issue if the same project uses find_package() for osgQt more than once in the source tree. This all is needed for OSG and not Qt, because Qt CMake find_package defines proper package names (e.g. Qt5::OpenGL) whereas the OpenSceneGraph CMake package only defines variables. This all helps with correct transitivity when doing a make install, then sharing to another computer, to avoid absolute paths being baked into the link library name.
• Added GNUInstallDirs usage. Without this on Linux your new install(TARGETS will copy the lib files to lib/ instead of lib64/. There may be other ways to solve this but this seemed most standard. Tested on Windows MSVC and Linux gcc.

Although I see the comments on your PR 63, I found this useful and thought I would expand on it based on how we expect to use osgQt.