message: Allow CUSTOM_EXTERNAL build type
diff: @@ -44,7 +44,7 @@ if (NOT CMAKE_BUILD_TYPE)
  set(CMAKE_BUILD_TYPE RELWITHDEBINFO)
  endif(NOT CMAKE_BUILD_TYPE)
  string(TOUPPER ${CMAKE_BUILD_TYPE} CMAKE_BUILD_TYPE)
  -set(CMAKE_ALLOWED_BUILD_TYPES DEBUG RELEASE RELWITHDEBINFO COVERAGE)
  +set(CMAKE_ALLOWED_BUILD_TYPES DEBUG RELEASE RELWITHDEBINFO COVERAGE CUSTOM_EXTERNAL)
  list(FIND CMAKE_ALLOWED_BUILD_TYPES "${CMAKE_BUILD_TYPE}" VALID_BUILD_TYPE)
  if(${VALID_BUILD_TYPE} EQUAL -1)
  string(REPLACE ";" ", " CMAKE_ALLOWED_BUILD_TYPES_FLAT "${CMAKE_ALLOWED_BUILD_TYPES}")
  