ADD_LIBRARY(${TARGET_TARGETNAME} MODULE ${TARGET_SRC} ${TARGET_H})
which gives them .so extensions. Since ".so" != ".dylib" osgDB::listAllAvailablePlugins finds no plug-ins. I believe the correct solution is to use CMAKE_SHARED_MODULE_SUFFIX instead. This builds and runs correctly on OS X but I have not tested on other platforms.
Attached is an updated src/osgDB/CMakeLists.txt based on rev 9915. The change is at line 108. To validate: build and then run bin/osgconv --formats. You should get many screenfuls of plug-in features, extensions and options."