Two small suggestions: 1. It would be nice if --help said what it was for. It seems to be for printing out all the names of modules that implement ISO C/POSIX functions, as suggested by its use in "Building the ISO C and POSIX substitutes". Adding a single line to --help would suffice.
2. When I run it, the first line of its output is blank; is this correct? If I understood what it does correctly, then "gnulib-tool -import `posix-modules`" would seem to be a nice half-way house between normal use of gnulib and the usage suggested "Building the ISO C and POSIX substitutes": i.e. rather than selecting individual modules, you just get all the POSIX/ISO C ones, and then just write your code to ISO/POSIX without worrying about what modules you need, but nonetheless modifying one's source code as required by gnulib. It would be nice to add something along these lines to the aforementioned chapter, or possibly elsewhere, if what I say is both correct and sensible. (Indeed, it seems to me that something like this should be recommended for any program which uses more than a few POSIX/ISO modules, as it makes use of gnulib much simpler, and means that as its coverage of POSIX and the various OSes for which it provides extra code grows, the program automatically becomes more portable, without the author having to add more modules.) -- http://rrt.sc3d.org