tests: fix maintainer-check-g++ make recipe

Clang++ issues warnings when it's used to compile C.  This make target
is precisely checking whether we can do that.

* configure.ac (NO_DEPRECATED_CXXFLAGS): New.
* tests/atlocal.in: Use it.

configure.ac

@@ -154,6 +154,8 @@ if test "$enable_gcc_warnings" = yes; then
   gl_WARN_ADD([-Wno-zero-as-null-pointer-constant], [FLEX_SCANNER_CXXFLAGS])
   CXXFLAGS=$save_CXXFLAGS
   gl_WARN_ADD([-fno-exceptions], [CXXFLAGS_NO_EXCEPTIONS])
+  # Clang++ deprecates compiling C.
+  gl_WARN_ADD([-Wno-deprecated], [WNO_DEPRECATED_CXXFLAGS])
   AC_LANG_POP([C++])
 fi