Consider updating minimum compiler versions to allow usage of C++14 features

[sethfowler]

C++14 adds a number of features that make it easier to write code that's clean, maintainable, and bug-free. The only problem: our current minimum GCC version is 4.8, which doesn't support most C++14 features.

GCC 5 and Clang 3.4 have complete support for C++14. On the GCC side, a majority of features are supported in GCC 4.9, though it seems to me that if we're bumping our supported compiler versions we may as well move to at least GCC 5. (Partly for ABI reasons; see below.)

I'd like to update the minimum compiler versions in the documentation if that won't cause issues for p4c developers or users.

@ChrisDodd, @ndramesh, @rsunkad: are you aware of any problems with making this change?

I suspect the most significant concern would be ABI incompatibilities; users may have issues if they need the ability to build an updated p4c binary and drop it into a system with an older libstdc++, since the libstdc++ ABI can and does change between versions. There are unfortunately some small ABI incompatibilities between the libstdc++ version associated with GCC 4.8 and the one associated with GCC 4.9, which is partly what motivates me to say that we may as well bump GCC to at least 5.

[mihaibudiu]

I think we should postpone this one. We risk losing users if we are too strict.

[sethfowler]

Actually I should've closed this a while ago. We decided to stick to C++11 only for now. Thanks for bringing it to my attention, Mihai.