Fix warning

I got many warnings during compilation of as-is master, so fixed them.

notable changes:

* 11fb8a0
    * checks return value of `sscanf()` to avoid "you're ignoring returned value" warning.  Because this is a change of direciton strict, there are some risk to false-negative return of `Qnil`.  But I guess it's enough safe and trivial condition.
* 6bd37f9
    * instead of `rb_thread_blocking_region()` emulation, do the same for `rb_thread_call_without_gvl`, that is introduced in Ruby 2.0.  This works in 1.8 too.

I checked all specs passed with all of listed version below:

    ruby 2.0.0p297 (2013-08-23 revision 42654) [x86_64-linux]
    ruby 1.9.3p392 (2013-02-22 revision 39385) [x86_64-linux]
    ruby 1.8.7 (2012-02-08 patchlevel 358) [x86_64-linux]