meson: convert po/

Meson warns if xgettext is not found.  In the future we may want to add
a required argument to i18n.gettext(); in the meanwhile, I am adding a
--enable-gettext/--disable-gettext option and feature detection in
configure.  This preserves QEMU's default behavior of detecting system
features, without any warning, if neither --enable-* nor --disable-*
is requested.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/meson.build b/meson.build
index 1ae9d42..62bf8a5 100644
--- a/meson.build
+++ b/meson.build
@@ -1115,6 +1115,9 @@
 subdir('pc-bios')
 subdir('tests')
 subdir('docs')
+if 'CONFIG_GTK' in config_host
+  subdir('po')
+endif
 
 if build_docs
   makeinfo = find_program('makeinfo', required: build_docs)