build: Require and check for gcc C++11 support

It is now 2017. We can and should be able to use C++11 features now.

Change-Id: I96477e4125390b17b43a3705bb1daf98fa01c9bb
Signed-off-by: Tom Tsou <tom.tsou@ettus.com>
diff --git a/configure.ac b/configure.ac
index 8ddd07d..8cfd25d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -36,6 +36,7 @@
 
 AM_PROG_AS
 AC_PROG_CXX
+AX_CXX_COMPILE_STDCXX_11
 AC_PROG_LN_S
 AC_PROG_MAKE_SET
 AC_PROG_INSTALL