OpenSystemsDevelopment / qpdf

Also add stubs for top-level QPDF methods (createFromJSON,
updateFromJSON)

Bidirectional base64; will be used by JSON v2.

It was being run at configuration time, not build time.

* 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 add a pkg-config runtime test that would have caught the error.

Some platforms need it for atomic<long long> but not for atomic<int>.