OpenSystemsDevelopment / qpdf

Allow QPDFObjectHandle::obj to be set prior resolving object.

ot_unresolved has been appended to the list object types in order to
preserve the output of existing test cases.

Report mem usage

Part of #729

Also, revert inlining of unparse and operator << from commit 4c6640c in
order to avoid exposing QUtil.

ABI breaking change

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>.