message: Move variable definition in if statement
diff: # Some functionnalities requires CMake 2.8.11 minimum
  if (NOT CMAKE_VERSION VERSION_LESS 2.8.11)
  - string(TIMESTAMP DATE_AND_TIME "%Y-%m-%d-%H-%M-%S")
  -
  # Definition of the custom target user_version
  add_custom_target(user_version)
  @@ -11,6 +9,7 @@ if (NOT CMAKE_VERSION VERSION_LESS 2.8.11)
  set(GUDHI_USER_VERSION_DIR ${CMAKE_CURRENT_BINARY_DIR}/${USER_VERSION_DIR})
  else()
  # set the GUDHI_USER_VERSION_DIR with timestamp and Gudhi version number
  + string(TIMESTAMP DATE_AND_TIME "%Y-%m-%d-%H-%M-%S")
  set(GUDHI_USER_VERSION_DIR ${CMAKE_CURRENT_BINARY_DIR}/${DATE_AND_TIME}_GUDHI_${GUDHI_VERSION})
  endif()
  