OpenSystemsDevelopment / qpdf

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.