Following statement will never set OpenSceneGraph_osgText_FOUND to TRUE:
```
FIND_PACKAGE(OpenSceneGraph REQUIRED COMPONENTS osgViewer osgText CONFIG)
```
It cause cmake failure, the error message is confusing:
```
Found package configuration file:
/usr/local/osg/gl2/lib/cmake/OpenSceneGraph/OpenSceneGraphConfig.cmake
but it set OpenSceneGraph_FOUND to FALSE so package "OpenSceneGraph" is
considered to be NOT FOUND.
```
I know one doesn't need to add osgText after osgViewer, but it should
not cause an error with confusing message.
