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.

tests/atlocal.in

@@ -42,6 +42,7 @@ GCC='@GCC@'
 # sets CFLAGS to this.
   NO_WERROR_CFLAGS='@CFLAGS@   @WARN_CFLAGS@   @WARN_CFLAGS_TEST@'
 NO_WERROR_CXXFLAGS='@CXXFLAGS@ @WARN_CXXFLAGS@ @WARN_CXXFLAGS_TEST@'
+WNO_DEPRECATED_CXXFLAGS='@WNO_DEPRECATED_CXXFLAGS@'
 
 # But most of the time, we want -Werror.
   CFLAGS="$NO_WERROR_CFLAGS   @WERROR_CFLAGS@"
@@ -99,7 +100,7 @@ if "$at_arg_compile_c_with_cxx"; then
   CC_IS_CXX=1
   CC=$CXX
   NO_WERROR_CFLAGS=$NO_WERROR_CXXFLAGS
-  CFLAGS=$CXXFLAGS
+  CFLAGS="$CXXFLAGS $WNO_DEPRECATED_CXXFLAGS"
   BISON_C_WORKS=$BISON_CXX_WORKS
 else
   CC_IS_CXX=0