We weren't setting a C++ version for build previously, which would
result in compiler specific default being used, most likely C++11
with GNU extensions on modern compilers and C++03 with GNU exts
on old compilers. This is bad because it potentially breaks build
on older toolchains that don't default to a modern C++.
Now we enable pure C++11 without GNU exts; this resulted in some
of the build breaking because of use of typeof() GNU C/C++ ext
in tests code, so fix that to use standard decltype() from C++11.
@fix
ac95f38d1b