Fix compilation on recent g++-9: require C++14.

So far, we have already been using C++14 features without explicitely
telling the compiler, so this just officially requires it during build.
Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

configure.ac

@@ -72,8 +72,8 @@ AC_ZITA_REQUIREMENTS
 AC_FFTW_CHECK
 AM_PATH_LIBGCRYPT
 
-# need c++11 mode
-AX_CXX_COMPILE_STDCXX_11(ext)
+# need c++14 mode
+AX_CXX_COMPILE_STDCXX_14(ext)
 
 # use -Wall
 AC_LANG_PUSH([C++])