cmake/build: cryptopp-modern support + packager-friendly flags - #169

Four small changes packagers asked for (FreeBSD ports has carried
local patches for these for years; rolling them upstream so the port
can drop them):

1. Move LICENSE.md from CMAKE_INSTALL_DATADIR to CMAKE_INSTALL_DOCDIR.
   GNUInstallDirs convention puts copyright/licence files under
   share/doc/<pkg>/ rather than share/<pkg>/, matching every distro's
   FHS expectation.

2. Make the file-preview fallback platform-aware. The legacy command
   was 'xterm -T "aMule Preview" -iconic -e mplayer "$file"' which
   assumed both xterm and mplayer were installed; on macOS, modern
   Linux desktops, and Windows that's no longer a reasonable
   assumption. Replace with:

     - macOS  : 'open "$file"'                  -- LaunchServices opens
                                                  the user's registered
                                                  application for the
                                                  file type
     - Windows: 'cmd /c start "" "$file"'       -- same via shell
                                                  associations
     - else   : 'xdg-open "$file"'              -- XDG MIME handler

   The warning text now interpolates the per-platform helper name via
   CFormat / "%s" so a new translatable string lands in the catalog.
   The Windows helper-name string is itself translatable (it's English
   prose); the macOS / xdg-open names stay as literal command names.
   Preferences -> Misc -> Video player still takes precedence
   regardless of platform.

3. Add -DENABLE_CCACHE=AUTO/ON/OFF. AUTO (default) preserves today's
   behaviour: use ccache if find_program locates it, log "ccache not
   found" otherwise. OFF skips the probe entirely -- distros that
   already manage ccache via their own wrapper (FreeBSD ports,
   Gentoo) don't want a second launcher chained in front. ON
   hard-fails the configure if ccache isn't found, so CI that
   *requires* ccache for cache-hit metrics fails fast instead of
   silently building uncached.

4. Add -DDEFAULT_VERSION_CHECK=ON/OFF. Plumbs a #cmakedefine01 macro
   into config.h that Preferences.cpp consumes for the initial state
   of the "Check for new aMule version" toggle on fresh installs.
   Default ON preserves today's behaviour for standalone / portable /
   AppImage builds. OFF is what OS-package builds (apt, brew, ports)
   typically want so the in-app updater doesn't compete with the
   distro's package manager. Existing users' saved amule.conf is
   unaffected -- the macro is only consulted at first-launch /
   config-reset.

docs/INSTALL.md gains rows for both flags.

scripts/update-po.sh refresh. Two semantic changes:

  1. The preview-warning msgid changed from
       "...preferences (default is mplayer)."
     to
       "...preferences (default is %s)." (now c-format)

  2. New msgid "the default Windows shell handler" -- the platform-
     specific helper-name substituted into %s on Windows.

Everything else is location-comment churn from line shifts in
DownloadListCtrl.cpp and Preferences.cpp.

The previous commit refreshed po/*.po via scripts/update-po.sh after
the preview-warning msgid changed from "...default is mplayer)." to
"...default is %s).". msgmerge correctly carried each old translation
into the new (now c-format) entry but flagged the entries fuzzy --
which would force every translator who'd already done the work to
re-tap "approved" in Weblate for what is, semantically, the same
sentence.

For every language whose existing translation contained the literal
token "mplayer", substitute it for "%s" and drop the fuzzy flag. At
runtime CFormat fills %s with the per-platform default ("xdg-open" on
Linux/BSD, "open" on macOS, the translated "the default Windows shell
handler" string on Windows), so the visible sentence reads naturally
in each locale.

Languages whose translator never mentioned mplayer literally, or
whose msgstr was empty, stay fuzzy so a human reviews them.

27/40 updated: ast, ca, de, el, es, et_EE, eu, fi, fr, gl, hu, it,
it_CH, lt, nl, nn, pl, pt_BR, pt_PT, ro, ru, sl, sv, tr, uk, zh_CN,
zh_TW.

13/40 stay fuzzy (empty or non-literal msgstr): ar, bg, bn, cs, da,
en_GB, he, hr, ja, ko_KR, lv, sq, ta.

The earlier commit in this series moves LICENSE.md from share/ to
share/doc/amule/ (FHS / GNUInstallDirs convention). The flatpak
manifests' top-level 'cleanup: - /share/doc' rule would now strip
the licence file from the final bundle along with the third-party
README/licence files it was originally targeting (cryptopp, boost,
libupnp).

Drop /share/doc from the cleanup list in both manifests we ship:

  - packaging/flathub/org.amule.aMule.yaml          (Flathub submission)
  - packaging/linux/flatpak/org.amule.aMule.yaml.in (local-build template;
                                                    the generated .yaml is
                                                    gitignored)

The bundle grows by a few KB of third-party doc files -- negligible
next to losing the licence. The .in template carries an inline
comment so a future reader doesn't re-add the rule; the Flathub
manifest stays comment-free per submission hygiene.