[rhythmbox] add missing AC_LANG_SOURCE to make autoconf shut up

[Jonathan Matthew <jmatthew src gnome org>]

commit 3b7b15205108ea3e7816b50281778689c234a7ab
Author: Jonathan Matthew <jonathan d14n org>
Date:   Tue Apr 16 07:57:49 2013 +1000

    add missing AC_LANG_SOURCE to make autoconf shut up

 configure.ac | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 5578a20..cf84d9b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -338,12 +338,12 @@ CFLAGS="$CFLAGS $X_CFLAGS"
 
 dnl Multimedia keys
 have_xfree=no
-AC_COMPILE_IFELSE([
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
        #include <X11/XF86keysym.h>
 int main(int argc,char **argv) {
   return 0;
 }
-], have_xfree=yes)
+]])], have_xfree=yes)
 AC_MSG_CHECKING(for XFree86 headers)
 AC_MSG_RESULT($have_xfree)
 if test x"$have_xfree" = "xyes" ; then