phosphor-user-manager: use c++17

Update configure.ac to choose the c++17 standard

Change-Id: I50e860687ee7b1e98c12f01e83acaad13c1fb2a9
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>
diff --git a/configure.ac b/configure.ac
index 6d41a5d..cf78609 100644
--- a/configure.ac
+++ b/configure.ac
@@ -50,7 +50,7 @@
 )
 
 # 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])
 
 # For libtools