Fix relative path to installation prefix in CMake config file.

CMakeLists.txt

@@ -162,7 +162,7 @@ endif ()
 install (TARGETS ${PACKAGE_NAME} DESTINATION ${LIBRARY_INSTALL_DIR} EXPORT gflags-lib)
 install (FILES   ${PUBLIC_HDRS}  DESTINATION ${INCLUDE_INSTALL_DIR}/${GFLAGS_NAMESPACE})
 
-file (RELATIVE_PATH INSTALL_PREFIX_REL2CONFIG_DIR "${CMAKE_INSTALL_PREFIX}" "${CMAKE_INSTALL_PREFIX}/${CONFIG_INSTALL_DIR}")
+file (RELATIVE_PATH INSTALL_PREFIX_REL2CONFIG_DIR "${CMAKE_INSTALL_PREFIX}/${CONFIG_INSTALL_DIR}" "${CMAKE_INSTALL_PREFIX}")
 configure_file (cmake/config.cmake.in  "${PROJECT_BINARY_DIR}/${PACKAGE_NAME}-config-install.cmake" @ONLY)
 configure_file (cmake/version.cmake.in "${PROJECT_BINARY_DIR}/${PACKAGE_NAME}-config-version.cmake" @ONLY)