OpenSystemsDevelopment / qpdf

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.

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.

Build fails on gcc 4.8 since version 9.1.1 and commit
752416554086d5d34323bc14164d5084db83cfbd:

libtests/cxx11.cc: In function 'void do_regex()':
libtests/cxx11.cc:347:42: error: 'strlen' is not a member of 'std'
     std::cregex_iterator m3(str7, str7 + std::strlen(str7), expr4);
                                          ^

To fix the build failure, add missing include on cstring

Fixes:
 - http://autobuild.buildroot.org/results/ad7fb68ae87850a85509eed80fd0cae8721b10c5

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

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