libcharset: Disable -Wsign-conversion warning inside libcharset

The code wasn’t written for it, and we don’t want to diverge from
upstream for this, so disable -Wsign-conversion in case it was set for
the overall GLib build.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3405

glib/libcharset/meson.build

@@ -1,6 +1,15 @@
+# glib enables various warnings which the libcharset code wasn’t designed to
+# work with
+extra_charset_args = cc.get_supported_arguments([
+  '-Wno-sign-conversion',
+])
+
 charset_lib = static_library('charset', 'localcharset.c',
   include_directories : configinc,
   pic : true,
-  c_args : [ '-DGLIB_CHARSETALIAS_DIR="@0@"'.format(glib_charsetaliasdir) ],
+  c_args : [
+    '-DGLIB_CHARSETALIAS_DIR="@0@"'.format(glib_charsetaliasdir),
+    extra_charset_args,
+  ],
   gnu_symbol_visibility : 'hidden',
 )