configure: Use c++17

This is needed to start using c++17 libraries and the code should
already be trivially compatible. We can't move away from
std::experimental::filesystem yet as that requires gcc8+.

Tested:
    Project still builds in the unit test environment although no tests
    are actually run.

Change-Id: I4db0bfe3d42be58d5296eddf5dea5383b7847374
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/configure.ac b/configure.ac
index 9930889..48adff4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -8,7 +8,7 @@
 
 
 # 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 programs.