Build as c++17

Setting as separate patch to not require move until user is ready.

Change-Id: Ia15de3ba1a4332dc51cc5539e895d7ab5ee072c7
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/configure.ac b/configure.ac
index e81c806..186c711 100644
--- a/configure.ac
+++ b/configure.ac
@@ -13,7 +13,7 @@
 AC_PROG_MAKE_SET
 
 # Checks for typedefs, structures, and compiler characteristics.
-AX_CXX_COMPILE_STDCXX_14([noext])
+AX_CXX_COMPILE_STDCXX_17([noext])
 AX_APPEND_COMPILE_FLAGS([-Wall -Werror], [CXXFLAGS])
 
 # Checks for libraries.