Extend the genex added by commit ca6ba3fe to accept a
;-list of paths, convert them all, and generate a list separated by the native shell `PATH``` separator.
$<SHELL_PATH:...> the parameter can now also be a list of paths in which case the paths will be separated by the OS dependent path separator. On linux the path separator is
':' and on Windows it is
';'. If the parameter is just one path the behaviour is identical to the current