Merge pull request #1930 from giacomofiorin/remove-lepton-cxx11-check

  file(GLOB COLVARS_SOURCES ${COLVARS_SOURCE_DIR}/[^.]*.cpp)

  # Build Lepton by default

  set(COLVARS_LEPTON_DEFAULT ON)

  # but not if C++11 is disabled per user request

  if(DEFINED DISABLE_CXX11_REQUIREMENT)

    if(DISABLE_CXX11_REQUIREMENT)

      set(COLVARS_LEPTON_DEFAULT OFF)

    endif()

  endif()

  option(COLVARS_LEPTON "Build and link the Lepton library" ${COLVARS_LEPTON_DEFAULT})

  # Verify that the user's choice is consistent

  if(DEFINED DISABLE_CXX11_REQUIREMENT)

    if((DISABLE_CXX11_REQUIREMENT) AND (COLVARS_LEPTON))

      message(FATAL_ERROR "Building the Lepton library requires C++11 or later.")

    endif()

  endif()

  option(COLVARS_LEPTON "Build and link the Lepton library" ON)

  if(COLVARS_LEPTON)

    set(LEPTON_DIR ${LAMMPS_LIB_SOURCE_DIR}/colvars/lepton)