nls: hard-fail when ENABLE_NLS=YES but gettext or libintl missing - #512

Mirror of #509 / #511 / #512 applied to cmake/boost.cmake. Two
soft-fail spots both flipped to FATAL_ERROR:

1. asio probe failed (boost found but check_include_files for
   boost/system/error_code.hpp + boost/asio.hpp couldn't compile),
   no DOWNLOAD_AND_BUILD_DEPS opt-in. This is the realistic case —
   a user has libboost-dev installed but missing some transitive
   header / version mismatch / link-step issue tripped the probe.
   Previously: silently set ENABLE_BOOST=FALSE, falling back to
   wxWidgets sockets. Now: FATAL_ERROR with hint to install full
   boost-dev, or pass -DENABLE_BOOST=NO, or pass
   -DDOWNLOAD_AND_BUILD_DEPS=YES.

2. Boost_FOUND=FALSE — defensive branch, practically unreachable
   because find_package(Boost CONFIG REQUIRED) at the top of the
   file already errors out when boost is absent. Replacing the
   silent disable with FATAL_ERROR makes the assumption explicit:
   if a future edit ever drops the REQUIRED keyword, this branch
   surfaces the regression instead of producing a green build
   with no boost.

The DOWNLOAD_AND_BUILD_DEPS escape hatch (where CMake is expected
to build Boost itself elsewhere in the configure pass) is preserved
for branch 1 — when DAaBD is set, no error fires.

Per @Vollstrecker on #507 (comment):

  > If a user sets a feature to enable and the deps for it aren't found,
  > we don't disable it and keep going, we error-out and the feature
  > will be disabled by the user, or the deps is provided.

This completes the four-file cmake/<feature>.cmake hard-fail series
(ip2country in #509, upnp in #511, nls in #512, boost here).

Test plan:

* configure with deps present (-DENABLE_BOOST=YES) — succeeds, boost
  found, ASIO_SOCKETS=TRUE (verified locally on macOS Homebrew
  boost 1.90.0).
* configure with -DENABLE_BOOST=NO — succeeds, no probing happens
  (top-level CMakeLists.txt:71 gates the include behind the option).
* configure with deps missing (-DENABLE_BOOST=YES, no boost) — the
  REQUIRED keyword on find_package(Boost CONFIG) at line 1 errors
  out before our changes are reached; the new FATAL_ERROR for the
  asio-probe-failed case isn't triggerable without artificially
  installing a broken boost. Mechanically identical FATAL_ERROR
  shape to #509 / #511 / #512.

…3nks

mrjimenez flagged on PR amule-project#514 review that these three contributors
were missing from the roster:

* Werner Mahr / Vollstrecker — has merge access on amule-project/amule
  and merges incoming PRs (e.g. amule-project#430, amule-project#385, amule-project#319, amule-project#330, amule-project#286). Goes
  under Maintainers based on that role.
* Pablo Barciela / Sc0w — recent contributor, e.g. wxWidgets 3.2.6
  build fixes, GTK version-check cleanup, dropping gtk1/gtk2 support,
  Chinese translation work. Goes under Developers.
* got3nks — Wayland app_id binding + SNI tray (amule-project#508), macOS UX
  (amule-project#508), libmaxminddb CI fix (amule-project#507), ip2country/upnp/nls/boost
  hard-fail series (amule-project#509/amule-project#511/amule-project#512/amule-project#513), packaging (amule-project#510),
  docs modernize (this PR). Goes under Developers.

Other recent folks may be missing too — those can be added in
follow-up commits as they're identified. mrjimenez explicitly noted
this is the priority subset to not block the rename PR.

…3nks

mrjimenez flagged on PR #514 review that these three contributors
were missing from the roster:

* Werner Mahr / Vollstrecker — has merge access on amule-project/amule
  and merges incoming PRs (e.g. #430, #385, #319, #330, #286). Goes
  under Maintainers based on that role.
* Pablo Barciela / Sc0w — recent contributor, e.g. wxWidgets 3.2.6
  build fixes, GTK version-check cleanup, dropping gtk1/gtk2 support,
  Chinese translation work. Goes under Developers.
* got3nks — Wayland app_id binding + SNI tray (#508), macOS UX
  (#508), libmaxminddb CI fix (#507), ip2country/upnp/nls/boost
  hard-fail series (#509/#511/#512/#513), packaging (#510),
  docs modernize (this PR). Goes under Developers.

Other recent folks may be missing too — those can be added in
follow-up commits as they're identified. mrjimenez explicitly noted
this is the priority subset to not block the rename PR.