A couple of tweaks to the build system.

* Clarify in config.hpp which parts are generated by the configure process.
* Include a library header in the compiler version checkers, so we can catch
  earlier if, e.g., the standard library is not usable (for instance when using
  libcxx with clang on some platforms without additional includes) before
  reaching the MPFR version check (which gives a confusing error message).