Build: Omit -Wmissing-noreturn from the default warnings.

It's not that important. It can be annoying in builds that
disable many features since in those cases the tests programs
will correctly trigger this warning with Clang.

configure.ac

@@ -1064,7 +1064,6 @@ if test "$GCC" = yes ; then
 			-Wold-style-definition \
 			-Wmissing-prototypes \
 			-Wmissing-declarations \
-			-Wmissing-noreturn \
 			-Wredundant-decls \
 			\
 			-Wc99-compat \