cmake CMAKE_<LANG>_SOURCE_FILE_EXTENSIONS examples

PaddlePaddle/Serving cmake/configure.cmake :150 

list(APPEND CMAKE_CXX_SOURCE_FILE_EXTENSIONS cu)

siconos/siconos cmake/SiconosTools.cmake :58 

foreach(_EXT
      ${CMAKE_CXX_SOURCE_FILE_EXTENSIONS}
      ${CMAKE_C_SOURCE_FILE_EXTENSIONS}
      ${CMAKE_Fortran_SOURCE_FILE_EXTENSIONS})