[sqlite] Allow ompilation without c RTL

[Arnaud Bouchez]

In order to compile SQLite3 without any dependency to the standard RTL for a 
language without it (Delphi, in my case), I had to make the following changes 
to the JSON1 extension:

  - fixed unknown isalnum/isdigit function calls:

#define safe_isdigit(x) sqlite3Isdigit((unsigned char)(x))
#define safe_isalnum(x) sqlite3Isalnum((unsigned char)(x))

  - use of strtod() converted to:

    case JSON_REAL: {
        double r;
        if (sqlite3AtoF(pNode->u.zJContent, &r, 
sqlite3Strlen30(pNode->u.zJContent), SQLITE_UTF8)==0){
         r = 0;
        }
      sqlite3_result_double(pCtx, r);
      break;
    }

It sounds better to use native sqlite3 functions in such case, as it is the 
case for all other SQlite3 code.

Hope it may find its path to the trunk.

Thanks for the good work!

Arnaud Bouchez
  RSI Video Technologies
  arnaud.bouchez at rsivideotech.com