Merge #7165: build: Enable C++11 in build, require C++11 compiler

7df9224 doc: Add note about new build/test requirements to release notes (Wladimir J. van der Laan)
2aacc72 build: update ax_cxx_compile_stdcxx to serial 4 (Wladimir J. van der Laan)
a398549 depends: use c++11 (Cory Fields)
67969af build: Enable C++11 build, require C++11 compiler (Wladimir J. van der Laan)