OpenSystemsDevelopment / qpdf

Some file names had `...` in their name, which causes problems on some
systems.

Specify qpdf libraries before external ones.
Specify LDFLAGS before libraries.
This should eliminate remaining cases of qpdf builds favoring
previously installed versions.

Use 0 instead of NULL in a cast.

Different compilers want different choices of headers for std::abs.
It's easier to just to not use it.

We were putting the value of LT_CURRENT in the versioned symbol map
instead of the soname major version, which was creating binary
incompatibility where none was present. Unfortunately, this bug is
already out there for qpdf 7.0.0, so we'll have to work around it for
now.

Add options to enable the raw encryption key to be directly shown or
specified. Thanks to Didier Stevens <didier.stevens@gmail.com> for the
idea and contribution of one implementation of this idea.

Make sure to link from the source tree before linking from the system.
In many environments, this is necessary to allow a newly built qpdf to
link properly instead of trying to link or resolve libraries from an
older installed version.

Fix comment, remove restriction that doesn't actually matter.