* build: Enable C++11 build, require C++11 compiler
Implements #6211.
* depends: use c++11
* build: update ax_cxx_compile_stdcxx to serial 4
* build: force a c++ standard to be specified
Newer compilers may switch to newer standards by default. For example, gcc6
uses std=gnu++14 by default.
* c++11: fix libbdb build against libc++ in c++11 mode
atomic_init clashes with
* c++11: CAccountingEntry must be defined before use in a list
c++11ism. This fixes builds against libc++.
