Fix some porting glitches found by Nelson H. F. Beebe.

* lib/abitset.c (abitset_resize): Rewrite to avoid warnings from
compilers that don't understand that abort () does not return.
* src/state.c (transitions_to): Likewise.
* m4/cxx.m4 (BISON_TEST_FOR_WORKING_CXX_COMPILER): Check
that '#include <cstdlib>' works.
* src/system.h (INT8_MIN, INT16_MIN, INT32_MIN, INT8_MAX):
(INT16_MAX, UINT8_MAX, INT32_MAX, UINT16_MAX, UINT32_MAX):
#undef if ! defined __STDC_VERSION__ || __STDC_VERSION__ < 199901,
for the benefit of some pre-C99 compilers.