warning: avoid warnings about unreachable code

Reported by Uxio Prego.
https://lists.gnu.org/archive/html/help-bison/2018-11/msg00031.html

We also need to move the unreachable 'goto' to a reachable place,
otherwise clang complains about the code being unreachable anyway.
See also https://bugs.llvm.org/show_bug.cgi?id=39736.

Interestingly, we don't have to apply that trick to
`#define YYCDEBUG if (false) std::cerr`, clang does not warn when the
code comes from macro expansion.

* configure.ac: Use -Wunreachable-code when supported.
* data/lalr1.cc, data/yacc.c: Pacify clang's warning about `if (0)`
by using a macro.
Another possibility was to move this statement to a reachable place.
* tests/actions.at, tests/c++.at: Avoid generating unreachable code.

configure.ac

@@ -113,6 +113,7 @@ if test "$enable_gcc_warnings" = yes; then
     -fno-color-diagnostics
     -Wno-keyword-macro'
 
+
   AC_LANG_PUSH([C])
   # Clang supports many of GCC's -W options, but only issues warnings
   # on the ones it does not recognize.  In that case, gl_WARN_ADD
@@ -126,6 +127,11 @@ if test "$enable_gcc_warnings" = yes; then
   # CFLAGS, and restore CFLAGS after the tests.
   save_CFLAGS=$CFLAGS
   gl_WARN_ADD([-Werror=unknown-warning-option], [CFLAGS])
+  # Accept this warning only if it is not too touchy (e.g., clang 3.3
+  # and 3.4).
+  gl_WARN_ADD([-Wunreachable-code], [WARN_CFLAGS],
+              [AC_LANG_PROGRAM([],
+              [[if (sizeof (long) < sizeof (int)) return 1;]])])
   for i in $warn_common $warn_c;
   do
     gl_WARN_ADD([$i], [WARN_CFLAGS])
@@ -147,6 +153,7 @@ if test "$enable_gcc_warnings" = yes; then
   CFLAGS=$save_CFLAGS
   AC_LANG_POP([C])
 
+
   AC_LANG_PUSH([C++])
   save_CXXFLAGS=$CXXFLAGS
   gl_WARN_ADD([-Werror=unknown-warning-option], [CXXFLAGS])
@@ -154,6 +161,11 @@ if test "$enable_gcc_warnings" = yes; then
   do
     gl_WARN_ADD([$i], [WARN_CXXFLAGS])
   done
+  # Accept this warning only if it is not too touchy (e.g., clang 3.3
+  # and 3.4).
+  gl_WARN_ADD([-Wunreachable-code], [WARN_CXXFLAGS],
+              [AC_LANG_PROGRAM([],
+              [[if (sizeof (long) < sizeof (int)) return 1;]])])
   gl_WARN_ADD([-Wzero-as-null-pointer-constant], [WARN_CXXFLAGS],
               [AC_LANG_PROGRAM([], [nullptr])])
   gl_WARN_ADD([-Werror], [WERROR_CXXFLAGS])