glr2.cc: disable GCC 4.6 warning

231. conflicts.at:1096: testing Syntax error in consistent error state: glr2.cc ...
    tests/conflicts.at:1096: $CXX $CXXFLAGS $CPPFLAGS  $LDFLAGS -o input input.cc $LIBS
    input.cc: In member function 'YYRESULTTAG glr_stack::yyresolveValue(glr_state*)':
    input.cc:2674:36: error: 'yysval' may be used uninitialized in this function [-Werror=uninitialized]

Do not initialize the variable: this way ASAN can really make sure we
do set it to a proper value.
If we initialize it, ASAN would report nothing.

* data/skeletons/c.m4 (YY_IGNORE_MAYBE_UNINITIALIZED_BEGIN): Disable
GCC 4.6's -Wuninitialized.
* data/skeletons/glr2.cc: Disable the warning locally.

data/skeletons/glr2.cc

@@ -2551,7 +2551,9 @@ public:
     if (yyflag == yyok)
       {
         yys->yyresolved = true;
+        YY_IGNORE_MAYBE_UNINITIALIZED_BEGIN
         yys->semanticVal() = yysval;
+        YY_IGNORE_MAYBE_UNINITIALIZED_END
       }
     else
       yys->setFirstVal(YY_NULLPTR);