Support Ruby 1.8.7 with rb_big_minus

Author: nurse

ext/mysql2/extconf.rb

@@ -23,6 +23,7 @@ def asplode(lib)
 have_func('rb_wait_for_single_fd')
 have_func('rb_hash_dup')
 have_func('rb_intern3')
+have_func('rb_big_cmp')
 
 # borrowed from mysqlplus
 # http://github.com/oldmoe/mysqlplus/blob/master/ext/extconf.rb

ext/mysql2/statement.c

@@ -204,6 +204,22 @@ static void set_buffer_for_string(MYSQL_BIND* bind_buffer, unsigned long *length
     xfree(length_buffers);                                  \
   }
 
+#ifndef HAVE_RB_BIG_CMP
+/* both x and y must be Bignum though MRI's rb_big_cmp support more types */
+static VALUE rb_big_cmp(VALUE x, VALUE y)
+{
+    /* Ruby 1.8.7 has rb_big_minus */
+    VALUE r = rb_big_minus(x, y);
+    if (FIXNUM_P(r)) {
+        long l = FIX2LONG(r);
+        return l < 0 ? -1 : l == 0 ? 0 : 1;
+    }
+    else { /* must be normalized Bignum */
+        return RBIGNUM_POSITIVE_P(r) ? 1 : -1;
+    }
+}
+#endif
+
 /* return 0 if the given bignum can cast as LONG_LONG, otherwise 1 */
 static int my_big2ll(VALUE bignum, LONG_LONG *ptr)
 {