def update_executables(elf):
pass<EOL>
Called at the beginning of customisation. Subclasses should
        override this if they need to set up the executables dictionary.

        Note that self.find_executables() is run afterwards, so the
        self.executables dictionary values can contain <F77> or <F90> as
        the command, which will be replaced by the found F77 or F90
        compiler.