SuperBuild: qt4 check variable exists before calling TO_NATIVE_PATH

SuperBuild/CMake/External_qt4.cmake

@@ -43,10 +43,15 @@ else()
   set(FT_INCLUDE ${_SB_FREETYPE_INCLUDE_DIRS})
 endif()
 
-file(TO_NATIVE_PATH ${FT_INCLUDE} FT_INCLUDE_NATIVE)
-file(TO_NATIVE_PATH ${SB_INSTALL_PREFIX} QT4_INSTALL_PREFIX_NATIVE)
-file(TO_NATIVE_PATH ${SB_INSTALL_PREFIX}/include QT4_INCLUDE_PREFIX_NATIVE)
-file(TO_NATIVE_PATH ${SB_INSTALL_PREFIX}/lib QT4_LIB_PREFIX_NATIVE)
+if(FT_INCLUDE)
+  file(TO_NATIVE_PATH ${FT_INCLUDE} FT_INCLUDE_NATIVE)
+endif()
+
+if(SB_INSTALL_PREFIX)
+  file(TO_NATIVE_PATH ${SB_INSTALL_PREFIX} QT4_INSTALL_PREFIX_NATIVE)
+  file(TO_NATIVE_PATH ${SB_INSTALL_PREFIX}/include QT4_INCLUDE_PREFIX_NATIVE)
+  file(TO_NATIVE_PATH ${SB_INSTALL_PREFIX}/lib QT4_LIB_PREFIX_NATIVE)
+endif()
 
 if(WIN32)
   set(QT4_BIN_EXT ".exe")