From Wang Rui, "A very small clerical error in the src/osgViewer/CMakeLists.txt makes MinGW/Cygwin builds fail. Now it is fixed and now OSG works well again with Cygwin. :-) The OSC plugin also needs a small fix to compile with Cygwin, as attached file src/osgPlugins/osc/osc/OscHostEndianness.h shows."