Fix a warning triggered in GCC (at least from 4.6 to 4.9):
input.cc: In constructor 'glr_stack_item::glr_stack_item(bool)':
input.cc:1371:5: error: declaration of 'is_state' shadows a member of 'this' [-Werror=shadow]
: is_state_(is_state)
^
* data/skeletons/glr2.cc (glr_stack_item): Alpha-convert.