[gdlmm] build: fix building on recent gtkmm

[Christian Hergert <chergert src gnome org>]

commit bc271a72fba51bfe64e0d14a5658e88db758bd18
Author: Christian Hergert <christian hergert me>
Date:   Fri Oct 16 18:45:45 2015 -0700

    build: fix building on recent gtkmm

 configure.ac |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index f973e45..b85a16c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -40,6 +40,7 @@ AC_PROG_CXX
 AC_DISABLE_STATIC
 AC_LIBTOOL_WIN32_DLL
 AC_PROG_LIBTOOL
+AX_CXX_COMPILE_STDCXX_11
 
 AC_SUBST([GDLMM_MODULES], ['gdl-3.0 >= 3.7 glibmm-2.4 >= 2.16 gtkmm-3.0 >= 3.0'])
 PKG_CHECK_MODULES([GDLMM], [$GDLMM_MODULES])