Array::join: Preallocate result String to avoid frequent resizing
When constructing a specific sized ReferenceCountedArray, don't eagerly allocate extra
Don't preallocate extra space in explicit size situations
Use -std=c++11 on GCC 4.7+
Prefer clang++ by default
Require std=c++11

Use AX_CXX_COMPILE_STDCXX_11 from https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx_11.html