Clarify g++ versions that are/aren't supported

[ahupowerdns]

I can report basic functionality on Linux using g++ 5.1, on 4.9 the C++ 2014 support is too weak it appears. This is all fine but it would be great to document it somewhere, it might lead people to think this is a clang project only.

Also, on Ubuntu 14.04 too I encountered the MHD unescape missing function.

[matt-42]

Thanks for the remark. Actually, just the sql_rest and sql_crud do not compile with g++ 5.3. It compiles all the rest of silicon without problem. It just takes 30-40% more time in average. I'll add a page on the documentation about the supported compilers.