Adding option to choose if octovis&dynamicEDT3D are built

As suggested in an issue I added the option to the main cmakelists :
BUILD_OCTOVIS_SUBPROJECT.
If it's ON (default) everything remains the same, if it's turned off the
octomap project can be built even if no QGLViewer and/or OpenGL are
found on the system.
It also can be used when octomap is used as an external project in a
cmake-project, the option can be passed in the cmake_args.

CMakeLists.txt

@@ -3,9 +3,12 @@ PROJECT( octomap-distribution )
 
 ENABLE_TESTING()  # enable CTest environment of subprojects
 
+option(BUILD_OCTOVIS_SUBPROJECT "Build targets from subproject octovis" ON)
+
 ADD_SUBDIRECTORY( octomap )
-ADD_SUBDIRECTORY( octovis )
-ADD_SUBDIRECTORY( dynamicEDT3D )
-
+if(BUILD_OCTOVIS_SUBPROJECT)
+	ADD_SUBDIRECTORY( octovis )
+	ADD_SUBDIRECTORY( dynamicEDT3D )
+endif()