Fixed XS_VERSION error while building on Perl 5.8.8

XS_VERSION is required by MY_CXT_KEY.

https://github.com/gfx/p5-Mouse/issues/18
ref. http://blog.64p.org/entry/2014/02/26/140302
Merge pull request #20 from tokuhirom/xs_version-5.8.8-fix

Fixed XS_VERSION error while building on Perl 5.8.8