[buildsystem] distribute cxx11_constexpr.m4

The check for constexpr was not copied into the distribution.
Unfortunately, the autotools will only warn about a missing macro, but
give valid results otherwise. This patch adds the cxx11_constexpr.m4 to
Makefile.am, so that it is distributed on "make dist".

m4/Makefile.am

@@ -19,6 +19,7 @@ ALLM4S = 					\
 	cxx0x_variadic_constructor_sfinae.m4    \
 	cxx11_initializer_list.m4		\
 	cxx11_conditional.m4			\
+	cxx11_constexpr.m4			\
 	dune.m4					\
 	dune_all.m4				\
 	dune_autobuild.m4			\