gettext: Respect the --subdir argument before the MESON_SUBDIR var
gettext: Respect the --subdir argument before the MESON_SUBDIR var

In the case of subproject we will properly setup the --subdir argument
but the MESON_SUBDIR is never adapted and will point to an empty string
leading to the following backtrace when building `gst-build`:

  msgfmt: error while opening "/home/thiblahute/devel/gstreamer/gst-build/af.po" for reading: No such file or directory
  Traceback (most recent call last):
    File "/home/thiblahute/devel/gstreamer/gst-build/meson/meson.py", line 37, in <module>
      sys.exit(main())
    File "/home/thiblahute/devel/gstreamer/gst-build/meson/meson.py", line 34, in main
      return mesonmain.run(launcher, sys.argv[1:])
    File "/home/thiblahute/devel/gstreamer/gst-build/meson/mesonbuild/mesonmain.py", line 248, in run
      sys.exit(run_script_command(args[1:]))
    File "/home/thiblahute/devel/gstreamer/gst-build/meson/mesonbuild/mesonmain.py", line 236, in run_script_command
      return cmdfunc(cmdargs)
    File "/home/thiblahute/devel/gstreamer/gst-build/meson/mesonbuild/scripts/gettext.py", line 111, in run
      if gen_gmo(src_sub, bld_sub, langs) != 0:
    File "/home/thiblahute/devel/gstreamer/gst-build/meson/mesonbuild/scripts/gettext.py", line 66, in gen_gmo
      '-o', os.path.join(bld_sub, l + '.gmo')])
    File "/usr/lib/python3.5/subprocess.py", line 581, in check_call
      raise CalledProcessError(retcode, cmd)
  subprocess.CalledProcessError: Command '['msgfmt', '/home/thiblahute/devel/gstreamer/gst-build/af.po', '-o', '/home/thiblahute/devel/gstreamer/gst-build/build/af.gmo']' returned non-zero exit status 1
  Failed to run install script: /usr/bin/python3 /home/thiblahute/devel/gstreamer/gst-build/meson/meson.py --internal gettext install --subdir=subprojects/gstreamer/po --localedir=share/locale --pkgname=gstreamer-1.0 --langs=af@@az@@be@@bg@@ca@@cs@@da@@de@@el@@en_GB@@eo@@es@@eu@@fi@@fr@@gl@@hr@@hu@@id@@it@@ja@@lt@@nb@@nl@@pl@@pt_BR@@ro@@ru@@rw@@sk@@sl@@sq@@sr@@sv@@tr@@uk@@vi@@zh_CN@@zh_TW
  FAILED: install
  '/usr/bin/python3' '/home/thiblahute/devel/gstreamer/gst-build/meson/meson.py' '--internal' 'install' '/home/thiblahute/devel/gstreamer/gst-build/build/meson-private/install.dat'

becm/meson,mesonbuild/meson,aaronp24/meson,aaronp24/meson,aaronp24/meson,ernestask/meson,jpakkane/meson,wberrier/meson,thiblahute/meson,QuLogic/meson,fmuellner/meson,QuLogic/meson,becm/meson,fmuellner/meson,MathieuDuponchelle/meson,ernestask/meson,jpakkane/meson,rhd/meson,jeandet/meson,ernestask/meson,thiblahute/meson,QuLogic/meson,QuLogic/meson,jpakkane/meson,pexip/meson,jpakkane/meson,jpakkane/meson,wberrier/meson,thiblahute/meson,ernestask/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,wberrier/meson,MathieuDuponchelle/meson,becm/meson,pexip/meson,jeandet/meson,pexip/meson,trhd/meson,ernestask/meson,fmuellner/meson,mesonbuild/meson,MathieuDuponchelle/meson,thiblahute/meson,trhd/meson,MathieuDuponchelle/meson,mesonbuild/meson,ernestask/meson,becm/meson,thiblahute/meson,MathieuDuponchelle/meson,aaronp24/meson,becm/meson,pexip/meson,QuLogic/meson,QuLogic/meson,wberrier/meson,fmuellner/meson,jeandet/meson,mesonbuild/meson,MathieuDuponchelle/meson,fmuellner/meson,aaronp24/meson,pexip/meson,jeandet/meson,trhd/meson,trhd/meson,rhd/meson,pexip/meson,QuLogic/meson,wberrier/meson,thiblahute/meson,aaronp24/meson,thiblahute/meson,rhd/meson,jeandet/meson,trhd/meson,mesonbuild/meson,rhd/meson,fmuellner/meson,pexip/meson,jeandet/meson,aaronp24/meson,fmuellner/meson,pexip/meson,becm/meson,rhd/meson,MathieuDuponchelle/meson,mesonbuild/meson,trhd/meson,ernestask/meson,jeandet/meson,QuLogic/meson,trhd/meson,becm/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,jeandet/meson,MathieuDuponchelle/meson,wberrier/meson,fmuellner/meson,trhd/meson,becm/meson,wberrier/meson,aaronp24/meson,ernestask/meson,fmuellner/meson,aaronp24/meson,QuLogic/meson,mesonbuild/meson,becm/meson,trhd/meson,pexip/meson,mesonbuild/meson,thiblahute/meson,jeandet/meson,becm/meson,wberrier/meson,thiblahute/meson,rhd/meson,ernestask/meson,rhd/meson,MathieuDuponchelle/meson,jpakkane/meson,rhd/meson