Ragel generates a code `0 <= (*p)` where `*p` is char.
As char is unsigned by default on arm and RISC-V, it is warned by gcc:
```
compiling parser.c
parser.c: In function ‘JSON_parse_string’:
parser.c:1566:2: warning: comparison is always true due to limited range of data type [-Wtype-limits]
if ( 0 <= (*p) && (*p) <= 31 )
^
parser.c:1596:2: warning: comparison is always true due to limited range of data type [-Wtype-limits]
if ( 0 <= (*p) && (*p) <= 31 )
^
```
This change removes the warning by substituting the condition with
`0 <= (signed char)(*p)`.
* ext/json/parser/prereq.mk (parser.c): use `enum` instead of
`static const int` to get rid of unused-const-variable warnings.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45356 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
