message: make GLUT an optional dependency
diff: find_package(Boost COMPONENTS filesystem iostreams system thread REQUIRED)
  find_package(GLEW REQUIRED)
  -find_package(GLUT REQUIRED)
  +find_package(GLUT)
  find_package(JPEG)
  find_package(OpenEXR)
  find_package(OpenGL REQUIRED)
  @@ -20,7 +20,9 @@ find_package(ZLIB)
  visionaray_use_package(Boost)
  visionaray_use_package(GLEW)
  +if (GLUT_FOUND)
  visionaray_use_package(GLUT)
  +endif()
  visionaray_use_package(JPEG)
  visionaray_use_package(OpenEXR)
  visionaray_use_package(OpenGL)
  @@ -203,7 +205,6 @@ set(COMMON_HEADERS
  tiff_image.h
  timer.h
  viewer_base.h
  - viewer_glut.h
  vsnray_loader.h
  )
  @@ -247,11 +248,15 @@ set(COMMON_SOURCES
  tga_image.cpp
  tiff_image.cpp
  viewer_base.cpp
  - viewer_glut.cpp
  vsnray_loader.cpp
  )
  +if (GLUT_FOUND)
  + set(COMMON_HEADERS ${COMMON_HEADERS} viewer_glut.h)
  + set(COMMON_SOURCES ${COMMON_SOURCES} viewer_glut.cpp)
  +endif()
  +
  if (APPLE AND VSNRAY_ENABLE_COCOA)
  set(COMMON_HEADERS ${COMMON_HEADERS} viewer_cocoa.h)
  set(COMMON_SOURCES ${COMMON_SOURCES} viewer_cocoa.mm)
  