build: Restore the ability to build gettext-tools separately, part 2.

build: Restore the ability to build gettext-tools separately, part 2.

* gettext-tools/configure.ac: New option --with-installed-csharp-dll.
(USE_INSTALLED_CSHARP_DLL): New conditional.
* gettext-tools/src/Makefile.am (CSHARP_DLL_DIR): New variable.
(msgunfmt.net.exe): Use it.
* configure.ac: Filter out --with-installed-csharp-dll from inherited --help
output.
* PACKAGING: Document that gettext-tools's configure needs to be invoked with
--with-installed-csharp-dll.