Allow CXX override during gem installation

TagLib 1.9.1 does not get along with clang++ on OS X 10.9, see
https://github.com/robinst/taglib-ruby/issues/44 . Overriding the C++
compiler used to compile taglib-ruby native extensions seems to solve
these problems.

Poking around in RbConfig feels like a hack but it is the best solution
I can come up with. I looked to Nokogiri for inspiration:

https://github.com/sparklemotion/nokogiri/blob/8f22560ab796e77a9a637c92cd5bf79261f1ad84/ext/nokogiri/extconf.rb#L299