OpenSystemsDevelopment / qpdf

Also, change so version to 0.

* Define it even though previous experiments have shown it to be
  unnecessary since it seems like it may be necessary after all
* Add QPDF_DLL_CLASS to QPDFObjectHelper and QPDFDocumentHelper in
  case there's some future unknown reason why someone may want to have
  them and/or in case it helps with the weird
  QPDFNameTreeObjectHelper problem.

Some new symbols were exported.

* 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

Run this:

for i in  **/*.cc **/*.c **/*.h **/*.hh; do
  clang-format < $i >| $i.new && mv $i.new $i
done

Fix C++ exception handling when -fvisibility=hidden

Ensure that QPDFExc and QPDFSystemError are marked visible, so that their typeinfo will not be
suppressed when -fvisibility=hidden.

Details:
    https://gcc.gnu.org/wiki/Visibility

The Apache License version 2.0 is now the primary license for qpdf.
However, users may, at their option, continue to use Artistic version
2.0.