meson: Add option to disable translations

This speeds up the meson install step by half a second
which, given the trivial changes required to add this
option, makes it worth the effort to support this.

Before:

```
‣ Running build script...
[1/418] Generating version.h with a custom command
Installing /root/build/po/be.gmo to /root/dest/usr/share/locale/be/LC_MESSAGES/systemd.mo
Installing /root/build/po/be@latin.gmo to /root/dest/usr/share/locale/be@latin/LC_MESSAGES/systemd.mo
Installing /root/build/po/bg.gmo to /root/dest/usr/share/locale/bg/LC_MESSAGES/systemd.mo
Installing /root/build/po/ca.gmo to /root/dest/usr/share/locale/ca/LC_MESSAGES/systemd.mo
Installing /root/build/po/cs.gmo to /root/dest/usr/share/locale/cs/LC_MESSAGES/systemd.mo
Installing /root/build/po/da.gmo to /root/dest/usr/share/locale/da/LC_MESSAGES/systemd.mo
Installing /root/build/po/de.gmo to /root/dest/usr/share/locale/de/LC_MESSAGES/systemd.mo
Installing /root/build/po/el.gmo to /root/dest/usr/share/locale/el/LC_MESSAGES/systemd.mo
Installing /root/build/po/es.gmo to /root/dest/usr/share/locale/es/LC_MESSAGES/systemd.mo
Installing /root/build/po/fr.gmo to /root/dest/usr/share/locale/fr/LC_MESSAGES/systemd.mo
Installing /root/build/po/gl.gmo to /root/dest/usr/share/locale/gl/LC_MESSAGES/systemd.mo
Installing /root/build/po/hr.gmo to /root/dest/usr/share/locale/hr/LC_MESSAGES/systemd.mo
Installing /root/build/po/hu.gmo to /root/dest/usr/share/locale/hu/LC_MESSAGES/systemd.mo
Installing /root/build/po/id.gmo to /root/dest/usr/share/locale/id/LC_MESSAGES/systemd.mo
Installing /root/build/po/it.gmo to /root/dest/usr/share/locale/it/LC_MESSAGES/systemd.mo
Installing /root/build/po/ja.gmo to /root/dest/usr/share/locale/ja/LC_MESSAGES/systemd.mo
Installing /root/build/po/ko.gmo to /root/dest/usr/share/locale/ko/LC_MESSAGES/systemd.mo
Installing /root/build/po/lt.gmo to /root/dest/usr/share/locale/lt/LC_MESSAGES/systemd.mo
Installing /root/build/po/pl.gmo to /root/dest/usr/share/locale/pl/LC_MESSAGES/systemd.mo
Installing /root/build/po/pt_BR.gmo to /root/dest/usr/share/locale/pt_BR/LC_MESSAGES/systemd.mo
Installing /root/build/po/ro.gmo to /root/dest/usr/share/locale/ro/LC_MESSAGES/systemd.mo
Installing /root/build/po/ru.gmo to /root/dest/usr/share/locale/ru/LC_MESSAGES/systemd.mo
Installing /root/build/po/sk.gmo to /root/dest/usr/share/locale/sk/LC_MESSAGES/systemd.mo
Installing /root/build/po/sr.gmo to /root/dest/usr/share/locale/sr/LC_MESSAGES/systemd.mo
Installing /root/build/po/sv.gmo to /root/dest/usr/share/locale/sv/LC_MESSAGES/systemd.mo
Installing /root/build/po/tr.gmo to /root/dest/usr/share/locale/tr/LC_MESSAGES/systemd.mo
Installing /root/build/po/uk.gmo to /root/dest/usr/share/locale/uk/LC_MESSAGES/systemd.mo
Installing /root/build/po/zh_CN.gmo to /root/dest/usr/share/locale/zh_CN/LC_MESSAGES/systemd.mo
Installing /root/build/po/zh_TW.gmo to /root/dest/usr/share/locale/zh_TW/LC_MESSAGES/systemd.mo
Installing /root/build/po/pa.gmo to /root/dest/usr/share/locale/pa/LC_MESSAGES/systemd.mo

real    0m1.467s
user    0m1.064s
sys     0m0.392s
```

After (with translations disabled):

```
‣ Running build script...
[1/418] Generating version.h with a custom command

real    0m0.925s
user    0m0.622s
sys     0m0.301s
```

1 files changed, 2 insertions, 0 deletions

diff --git a/meson_options.txt b/meson_options.txt
index 6e104d21c3..d1096655dc 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -161,6 +161,8 @@ option('man', type : 'combo', choices : ['auto', 'true', 'false'],
 option('html', type : 'combo', choices : ['auto', 'true', 'false'],
        value : 'false',
        description : 'build and install html pages')
+option('translations', type : 'boolean', value : true,
+       description : 'build and install translations')
 
 option('certificate-root', type : 'string', value : '/etc/ssl',
        description : 'the prefix for TLS certificates')