Proposal/no cxx14 macros

I also propose BOOST_RELAXED_CONSTEXPR, which will help Fusion's constexpr supports: [Test output: BP x86_64 C++11 - fusion - swap / clang-linux-3.4~c11_libc++](http://www.boost.org/development/tests/develop/developer/output/BP%20x86_64%20C++11-boost-bin-v2-libs-fusion-test-swap-test-clang-linux-3-4~c11_libc++-debug-debug-symbols-off.html)

New macros:
- BOOST_NO_CXX14_BINARY_LITERALS
- BOOST_NO_CXX14_RETURN_TYPE_DEDUCTION
- BOOST_NO_CXX14_DECLTYPE_AUTO
- BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURE
- BOOST_NO_CXX14_GENERIC_LAMBDAS
- BOOST_NO_CXX14_DEPRECATED_ATTRIBUTES
- BOOST_NO_CXX14_DIGIT_SEPARATOR
- BOOST_NO_CXX14_VARIABLE_TEMPLATES
- BOOST_NO_CXX14_RELAXED_CONSTEXPR
- BOOST_NO_CXX14_MEMBER_INITIALIZERS_AND_AGGREGATES
- BOOST_NO_CXX14_SIZED_DEALLOCATION
- BOOST_RELAXED_CONSTEXPR
since return-type deduction is arguably one of the two big features of C++14 for designing template libraries (along with generic lambdas), I think it's important to make it available as early as possible to users.

As a library author, I'd like it if GCC 4.8 was enough, in particular because 4.9 isn't as widely deployed in stable linux distributions.