When testing compiler modes higher than C++11 for constructs we need,
include a check for using `unique_ptr` in that mode. The PGI 18.4
compiler in some environments supports `unique_ptr` in C++11 mode
but is broken for C++14 and C++17. Check that `unique_ptr` works
in these modes before using one.
The check for C++14 and cstdio is a special case of the more general
problem of checking that the compiler's C++14 mode supports everything
we need. Rename the checks accordingly.