Document gnulib-l10n and other dependencies

Simon reports in https://lists.gnu.org/archive/html/bug-gnulib/2025-03/msg00021.html that libidn2 makes use of gnulib-l10n. This ought to be documented, so that

  • distributors put a dependency from libidn2 to gnulib-l10n in their packaging system,
  • users who install the package from source know what value to pass in the --with-gnulib-prefix option.

Here's a take at it:

  1. Add a file DEPENDENCIES.

DEPENDENCIES.gz

  1. Fill in the two Needed for <...>. placeholders.

  2. Reference this file from the README, removing the paragraphs

The Libidn2 library may use GNU libunistring for Unicode processing
and GNU libiconv for character set conversion.  It is recommended to
install them before building and installing libidn2.  See the
following links for more information on these packages:

Unistring
iconv

The iconv dependency is optional -- it is required for the functions
involving locale to UTF8 conversions -- but is recommended.
When the recommended libunistring is not available, libidn2 uses
internal replacement functionality which increases the size of the
library.  To use the internal libunistring-replacement rather than the
system libunistring (even when deemed to be sufficient) you may use:

./configure --with-included-libunistring
  1. In Makefile.am, add DEPENDENCIES to the EXTRA_DIST variable.