The old version worked like this:

fails_with :gcc => '4.8.1'

That wasn't really flexible enough, and made it harder to distinguish
different releases in the same GCC series. Since no one was really
using it yet, this adjusts the syntax to be more similar to the
Apple compilers:

fails_with :gcc => '4.8' do
  release '4.8.1'
end

Like with Apple compilers, omitting `release` blacklists the entire
series.

This also unifies the `build` and `version` attributes and accessors,
and exposes them under both names.

This adds support for non-Apple GCC compilers in the fails_with code.
A fails_with block for a non-Apple compiler looks like:

fails_with :gcc => '4.8.1' do
  cause 'Foo'
end

Non-Apple compilers don't have build numbers, so compiler failures are
based on version strings instead.

Internally non-Apple compilers can be distinguished because they are
passed around as strings instead of symbols.

In addition, this alters the priority list for compilers, with the
following changes:

* Apple GCC 4.2 and LLVM-GCC swap positions, with GCC now taking
  priority. (Maybe LLVM-GCC should just go away.)
* Non-Apple GCC compilers are ranked below GCC 4.2 but above LLVM-GCC
  and Apple GCC 4.0.