Oops: removed now-obsolete warning from NEWS.

Updated build docs for CMake.

Allow build to hide std::experimental::optional.

Introduces a new macro, `PQXX_HIDE_EXP_OPTIONAL`, to hide support for
`std::experimental::optional` while building code for linking against
libpqxx.  This can paper over a difference in selected C++ language
versions between the build of libpqxx itself, and the build of the
software which uses it.

Fixes jtv#93.
Fixes jtv#95.

Regenerate on Ubuntu 17.10.

Documentation fix.

Update version to 6.2.1.

Bump version to 6.2.0.

The addition of the library/headers version check introduces an ABI
compatibility.

Bump version to 6.1.1.

Revert export of row constructor.

Turns out PQXX_LIBEXPORT was _not_ the solution to the problem where
the constructor was reported as a missing symbol.  Worse, it breaks on
Visual Studio as well.

Re-generate include/Makefile.in on Ubuntu 17.10.