OpenSystemsDevelopment / qpdf

This reverts commit dc059560e73e0b373a6e54e71b07e3af4b692cb4.

I changed my mind. There's no harm in leaving it deprecated for a
release cycle.

See ChangeLog for rationale for not deprecating it as originally
planned.

Remove the name/number tree object helper constructors that don't take
a QPDF&.

* Change DLL_EXPORT to libqpdf_EXPORTS (internal to the build). The
  new name is cmake's default, is more conventional, and is less
  likely to clash with other symbols.
* Add QPDF_DLL_PRIVATE for non-Windows
* Make logic around when to define QPDF_DLL et al more explicit
* Add detailed comments

Also update to cmake variable rather than autoconf variable

Some people don't know what CI is, so remove use of that abbreviation
when it doesn't add any value.

Sometimes someone may build only a subset of the documentation. The
logic has to be conditional upon whether it exists, not whether it is
built, since we pull pre-built docs in for CI installation.