use the rubyarchhdrdir from RbConfig when present

It defines where the arch-specific header files are. This commit fixes
installing the rice gem on Debian when using Debian's prepackaged ruby2.0
package, where config.h is in /usr/include/ARCH/ruby, not /usr/include/ruby/ARCH
as expected by the current version of ruby.ac
Merge pull request #68 from doudou/master

use the rubyarchhdrdir from RbConfig when present