OpenSystemsDevelopment / qpdf

This was added in C++11.

If a page contains a form XObject, also filter the form XObject and
remove its unreferenced resources.

Refactor removeUnreferencedResources to prepare for filtering form
XObjects.

fix-qdf assumes there is exactly one XRef stream and that it is at the
end of the file.

If the value of /CS in the inline image dictionary was is key in the
page's /Resource -> /ColorSpace dictionary, properly resolve it by
referencing the proper colorspace, and not just the name, in the
external image dictionary.

Fix is based on guidance
https://support.microsoft.com/en-us/help/238187/cryptacquirecontext-use-and-troubleshooting
and is the proper fix for #285/#286

This commit adds the preserve_eol flags but doesn't implement EOL
preservation yet.

Even though this is disallowed by the spec, files like this have been
encountered in the wild.

Thanks to Zdenek Dohnal <zdohnal@redhat.com> for contributing the code
used for the gnutls crypto provider.

Just rename MD5 -> MD5_native in place so that git annotate will show
the lines as having originated there.

Includes updates to m4/ax_cxx_compile_stdcxx.m4 to make it work with
msvc, which supports C++-11 with no flags but doesn't set __cplusplus
to a recent value.

Various PDF digital signing tools do not encrypt /Contents value in
signature dictionary. Adobe Acrobat Reader DC can handle a PDF with
the /Contents value not encrypted.

Write Contents in signature dictionary without encryption

Tests ensure that string /Contents are not handled specially when not
found in sig dicts.