CMake Install: Capitalization fixes for config.cmake file

[emminizer]

Minor fixes for CMake installation support on Linux

• Minimum version fixed from 3.10 to 3.11 to avoid constant warnings about GL on configure
• Added missing Threads dependency for UNIX systems
• Capitalization fix: osgEarth-config.cmake to osgearth-config.cmake, so that find_package(osgEarth) can work on Linux
• Filename rename from osgEarth-configVersion.cmake, to osgearth-config-version.cmake, to match style of other file
• Install config files to share/osgEarth instead of share/osgearth, matching install location for shaders

Tested with a small Linux app that uses find_package(osgEarth). It did still require explicit links to GDAL and GEOS in the final application (Linux only) but it found the package and I was able to link successfully. Before this change, CMake could not be coaxed to find the package because of the capitalization in both share/osgEarth and osgEarth-config.cmake being inconsistent with what CMake expected.

[plevy]

Dan, sorry about this, there were two cmake related PRs and the other was a bigger change, switching over to using the GNUInstallDirs. I merged that change, tried to make the changes from your PR and still might need a few more changes. Can you take a look at master and see what else needs to be done to get you back on track with the lower case changes.

Again, sorry for making you go over this again