Build as c++17

Set configure.ac to attempt to build with c++17.

Change-Id: Ie09177e5de43bb1789befbb32678b802c1959bd2
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/configure.ac b/configure.ac
index cc8c655..fea80cd 100644
--- a/configure.ac
+++ b/configure.ac
@@ -19,7 +19,7 @@
 AX_PTHREAD([], [AC_MSG_ERROR(["pthread required and not found"])])
 
 # Checks for typedefs, structures, and compiler characteristics.
-AX_CXX_COMPILE_STDCXX_14([noext])
+AX_CXX_COMPILE_STDCXX_17([noext])
 AX_APPEND_COMPILE_FLAGS([-fpic -Wall -Werror], [CXXFLAGS])
 
 # Checks for header files.