Add USE_QT4 to allow building with Qt4 when Qt5 also present

[cjmayo]

I'm building osgEarth to support the Globe plugin for QGIS. QGIS stable is still recommending Qt4 at this time but I also have Qt5 installed. Currently osgEarth will automatically build for Qt5 for me.

This is my quick solution, adding a USE_QT4 option. Submitting here in case it is useful for others.

Without USE_QT4 it will still build for Qt5 if installed. It won't automatically fallback from trying Qt5 to try Qt4 - arguably that's an improvement.
Happy to change the logic, create a OSGEARTH_USE_QT4? etc. Possibly CMake should fail if OSGEARTH_USE_QT is set and no Qt is found.

[manisandro]

Globe maintainer here. I'd like to go further with this and actually allow side-by-side installation of the qt4 and qt5 variants (to allow developing both qgis-2.x and qgis-3.x without library chaos). I've got a patch which, in addition to allowing to choose the Qt version via USE_QT5={true/false}, changes the names of the osgEarthQt library and the qt applications to allow installing both variants without conflicts. The patch is here: https://github.com/manisandro/osgearth/commit/80901d4d9e07bf7a271e9da23fbc8ed9b6c2e909
@cjmayo Shall I open a new PR or do you want to integrate these changes into this PR?

A similar patch I've proposed to osgQt has been merged: openscenegraph/osgQt#3

[gwaldron]

You can integrate the changes, but please take care with the versioning. osgEarth must be able to build against OSG 3.4 up through the master branch.

[cjmayo]

@manisandro I've tested (osgEarth 2.8.0 with OpenSceneGraph 3.4.0) manisandro@80901d4 for my case of both Qt4 and Qt5 installed and only building for Qt4 and as it does that and more happy to close this one.

[manisandro]

Just for info, I'm waiting for openscenegraph/osgQt#4 and then I'll prepare the PR for osgearth.