OpenSystemsDevelopment / qpdf

This is all that ever worked. The test suite was trying to do
something different from ClosedFileInputSource.

Use autoconf rather than ifdefs to determine what format string to use
for long long.

Preparing to migrate qpdf's main build/CI to GitHub actions

It is redundant with qpdf's main CI environment, which is Azure
Pipelines, but may soon be migrated to GitHub Actions.

Make clear that you almost always want + or - before an angle when
specifying rotation.

When possible, use `for (auto&` or `for (auto const&` when iterating
using C++-11 style iterators.

Also switch to colon-style iteration in some cases. Thanks to Dean
Scarff for drawing this to my attention after detecting some
unnecessary copies with
https://clang.llvm.org/extra/clang-tidy/checks/performance-for-range-copy.html

A class can't have a PointerHolder to itself since PointerHolder
doesn't have the concept of weak references.

This is needed for some compilers on some platforms, notably current
versions of clang.