Silence a warning if pkg_config is not installed.

configure

@@ -761,7 +761,7 @@ check_pkg_config(){
     headers="$2"
     funcs="$3"
     shift 3
-    $pkg_config --exists $pkg || return
+    $pkg_config --exists $pkg 2>/dev/null || return
     pkg_cflags=$($pkg_config --cflags $pkg)
     pkg_libs=$($pkg_config --libs $pkg)
     check_func_headers "$headers" "$funcs" $pkg_cflags $pkg_libs "$@" &&