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A general description is here.
There is now support for parallelized Link-Time Optimization (LTO) with GCC and for ‘thin’ LTO with ‘clang’ _via_ setting the ‘LTO’ macro.
There is support for setting a different LTO flag for the Fortran compiler, including to empty when mixing ‘clang’ and ‘gfortran’ (as on macOS). See file ‘config.site’.
There is a new ‘LTO_LD’ macro to set linker options for LTO compilation, for example to select an alternative linker or to parallelize thin LTO.