BUG: Don't force Cocoa or Carbon when using X11 on Mac.

If building with VTK_USE_X on Mac we will get compile errors
if we force Cocoa or Carbon ON.

CMakeLists.txt

@@ -306,8 +306,8 @@ IF(VTK_USE_QT)
   IF(NOT QT4_FOUND)
     MESSAGE(SEND_ERROR "Qt ${QT_MIN_VERSION} or greater not found.  Please check the QT_QMAKE_EXECUTABLE variable.")
   ELSE(NOT QT4 FOUND)
-    # enforce Carbon or Cocoa in VTK for Qt/Mac
-    IF(Q_WS_MAC)
+    # enforce Carbon or Cocoa in VTK for Qt/Mac if we are not building with X11 explicitly turned on
+    IF(Q_WS_MAC AND NOT VTK_USE_X)
       IF(QT_MAC_USE_COCOA)
         SET(VTK_USE_CARBON OFF CACHE BOOL "Build VTK with Carbon" FORCE)
         SET(VTK_USE_COCOA ON CACHE BOOL "Build VTK with Cocoa" FORCE)
@@ -315,7 +315,7 @@ IF(VTK_USE_QT)
         SET(VTK_USE_CARBON ON CACHE BOOL "Build VTK with Carbon" FORCE)
         SET(VTK_USE_COCOA OFF CACHE BOOL "Build VTK with Cocoa" FORCE)
       ENDIF(QT_MAC_USE_COCOA)
-    ENDIF(Q_WS_MAC)
+    ENDIF(Q_WS_MAC AND NOT VTK_USE_X)
   ENDIF(NOT QT4_FOUND)
 ENDIF(VTK_USE_QT)