switching to std::unique_ptr<> in 52c9f62 caused some trouble for
users of "older" compilers: they needed to add the --std=cxx11 flag
by hand. to make life more convenient, m4/ax_cxx_compile_stdcxx.m4
and m4/ax_cxx_compile_stdcxx_11.m4 are added to do the right thing.
in my tests, the last "good" compiler is gcc-4.8, gcc-4.7 does not
support the required c++11 standard.
clang++-3.5 also works.
