regcomp.c: make non-_LIBC implementation of build_range_exp consistent

regcomp.c: make non-_LIBC implementation of build_range_exp consistent

The _LIBC implementation of build_range_exp correctly honors the
RE_NO_EMPTY_RANGES flag when checking for reversed range endpoints.
However, the non-_LIBC implementation would ignore that syntax-bit
flag and return REG_ERANGE unconditionally.
This change makes it honor that flag.
* lib/regcomp.c (build_range_exp) [!_LIBC]: Add a parameter: "syntax".
Make two pointer parameters "const".
Use "syntax" bits in order to honor RE_NO_EMPTY_RANGES.
(parse_bracket_exp): Update caller.