OpenSystemsDevelopment / qpdf

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.

This uses C++-11 thread-safe static initializers now.

Explicitly enable openssl crypto provider in the linux and sanitizer
builds, and disable it in the windows build.

Fixes qpdf/qpdf#417

Issue #399 mentioned a use case for which qpdf has support, but the
fact that it is supported was not documented or in the test suite,
making it vulerable to accidental breakage.

StreamDataProvider::provideStreamData now has a rich enough API for it
to effectively proxy to pipeStreamData.

I keep looking here to find out when I started requiring C++-11, and
now I will find the answer in the first place I look.

Fixes qpdf/qpdf#419

This was being saved until we had to break ABI.