[F] Added checks for building assimp_qt_viewer.

CMakeLists.txt

@@ -306,7 +306,32 @@ IF ( ASSIMP_BUILD_ASSIMP_TOOLS )
   ENDIF ( WIN32 )
 
   ADD_SUBDIRECTORY( tools/assimp_cmd/ )
-  ADD_SUBDIRECTORY( tools/assimp_qt_viewer/ )
+
+  # Check dependencies for assimp_qt_viewer.
+  # Why here? Maybe user do not want Qt viewer and have no Qt.
+  # Why assimp_qt_viewer/CMakeLists.txt still contain similar check?
+  # Because viewer can be build independently of Assimp.
+  FIND_PACKAGE(Qt4 QUIET)
+  FIND_PACKAGE(DevIL QUIET)
+  FIND_PACKAGE(OpenGL QUIET)
+  IF ( Qt4_FOUND AND IL_FOUND AND OPENGL_FOUND)
+    ADD_SUBDIRECTORY( tools/assimp_qt_viewer/ )
+  ELSE()
+    SET ( ASSIMP_QT_VIEWER_DEPENDENCIES "")
+    IF (NOT Qt4_FOUND)
+      SET ( ASSIMP_QT_VIEWER_DEPENDENCIES "${ASSIMP_QT_VIEWER_DEPENDENCIES} Qt4")
+    ENDIF (NOT Qt4_FOUND)
+
+    IF (NOT IL_FOUND)
+      SET ( ASSIMP_QT_VIEWER_DEPENDENCIES "${ASSIMP_QT_VIEWER_DEPENDENCIES} DevIL")
+    ENDIF (NOT IL_FOUND)
+
+    IF (NOT OPENGL_FOUND)
+      SET ( ASSIMP_QT_VIEWER_DEPENDENCIES "${ASSIMP_QT_VIEWER_DEPENDENCIES} OpengGL")
+    ENDIF (NOT OPENGL_FOUND)
+
+    MESSAGE (WARNING "Build of assimp_qt_viewer is disabled. Unsatisfied dendencies: ${ASSIMP_QT_VIEWER_DEPENDENCIES}")
+  ENDIF ( Qt4_FOUND AND IL_FOUND AND OPENGL_FOUND)
 ENDIF ( ASSIMP_BUILD_ASSIMP_TOOLS )
 
 option ( ASSIMP_BUILD_SAMPLES