[sysprof] build: use -std=gnu11 by default

[Christian Hergert <chergert src gnome org>]

commit d3a4355a97bc7694474de704f7ce5c639f3fe0b4
Author: Christian Hergert <chergert redhat com>
Date:   Tue Aug 23 15:42:22 2016 -0700

    build: use -std=gnu11 by default
    
    We require C11 for stdatomic.h

 configure.ac |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 1004182..a455bfe 100644
--- a/configure.ac
+++ b/configure.ac
@@ -91,6 +91,14 @@ AC_CHECK_MEMBERS([struct perf_event_attr.use_clockid, struct perf_event_attr.clo
 
 
 dnl ***********************************************************************
+dnl Ensure C11 is Supported
+dnl ***********************************************************************
+AX_CHECK_COMPILE_FLAG([-std=gnu11],
+                      [CFLAGS="$CFLAGS -std=gnu11"],
+                      [AC_MSG_ERROR([C compiler cannot compile GNU C11 code])])
+
+
+dnl ***********************************************************************
 dnl Check for required packages
 dnl ***********************************************************************
 m4_define([glib_required_version], [2.44.0])