glr2.cc: avoid type-punning issues

glr2.cc: avoid type-punning issues

On the CI, tests fail with GCC 4.6 to GCC 6 as follows:

    tests/synclines.at:440: COLUMNS=1000; export COLUMNS; NO_TERM_HYPERLINKS=1; export NO_TERM_HYPERLINKS;  bison --color=no -fno-caret  -o \"\\\"\".cc \"\\\"\".y
    tests/synclines.at:440: $CXX $CXXFLAGS $CPPFLAGS  $LDFLAGS -o \"\\\"\" \"\\\"\".cc $LIBS
    stderr:
    "\"".cc: In member function 'glr_state& glr_stack_item::getState()':
    "\"".cc:1404:47: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
         return *reinterpret_cast<glr_state*>(&raw_);
                                                   ^
    "\"".cc: In member function 'const glr_state& glr_stack_item::getState() const':
    "\"".cc:1408:53: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
         return *reinterpret_cast<const glr_state*>(&raw_);
                                                         ^
    "\"".cc: In member function 'semantic_option& glr_stack_item::getOption()':
    "\"".cc:1413:53: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
         return *reinterpret_cast<semantic_option*>(&raw_);
                                                         ^
    "\"".cc: In member function 'const semantic_option& glr_stack_item::getOption() const':
    "\"".cc:1417:59: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
         return *reinterpret_cast<const semantic_option*>(&raw_);
                                                           ^

See also be6fa942acae21a4a025bac5e339451be6ad136d.

* data/skeletons/glr2.cc (glr_stack_item): Use a temporary void*
variable to avoid type-punning issues with reinterpret_cast.