It is working now! Thank you so much!
Scratch the text below here.

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EDITED

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I did as you requested, but it created a new error:

g++ -c -pipe -O2 -std=gnu++11 -Wall -W -D_REENTRANT -fPIC -DSERVER_RELEASE -DNOMINMAX -DYACREADER_LIBRARY -DQT_NO_DEBUG_OUTPUT -DNO_OPENGL -DLIBDIR=\"/usr/lib\" -DDATADIR=\"/usr/share\" -DBINDIR=\"/usr/bin\" -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_SQL_LIB -DQT_NETWORK_LIB -DQT_CORE_LIB -I. -I../YACReaderLibrary -I../common -I../YACReaderLibrary/server -I../YACReaderLibrary/db -I../YACReaderLibrary/server -I../YACReaderLibrary/server/lib/bfLogging -I../YACReaderLibrary/server/lib/bfHttpServer -I../YACReaderLibrary/server/lib/bfTemplateEngine -I../QsLog -isystem /usr/include/poppler/qt5 -isystem /usr/include/poppler -isystem /usr/include/arm-linux-gnueabihf/qt5 -isystem /usr/include/arm-linux-gnueabihf/qt5/QtGui -isystem /usr/include/arm-linux-gnueabihf/qt5/QtSql -isystem /usr/include/arm-linux-gnueabihf/qt5/QtNetwork -isystem /usr/include/arm-linux-gnueabihf/qt5/QtCore -I. -I/usr/lib/arm-linux-gnueabihf/qt5/mkspecs/linux-g++ -o library_creator.o ../YACReaderLibrary/library_creator.cpp
../YACReaderLibrary/library_creator.cpp:15:32: fatal error: compressed_archive.h: No such file or directory

^
compilation terminated.
Makefile:897: recipe for target 'library_creator.o' failed
make: *** [library_creator.o] Error 1

No need to thank me - I'm the guy that broke it in the first place …

Problem compiling on dietpi v162.
The qmake YACReaderLibrarServer.pro fails because missing poppler-qt5
But it's installed.
Searching the error using debug option on qmake, it fails on ../dependencies/pdf_backend.pri, because it can't found /usr/lib/libpoppler-qt5
With dietpi v162, stretch based, those libraries are located at /usr/lib/arm-linux-gnueabihf, so I did:

ln -s /usr/lib/arm-linux-gnueabihf /usr/lib/libpoppler-qt5

and with this link, I can qmake and the rest of proccess to compile.

This is a known buildsystem bug for YACReader 9.0.0 on Debian based systems. This is the problem:

- Debian ships a version of qmake which does not support pkg-config
- Debian ships shared libraries in architecture-dependend /usr/lib/architecture
- My fallback solution for missing pkg-config is too strict

Fixes for this are already included in my Debian packages and on the "default" branch on Luis' Bitbucket repository (merged today, so you might need to run 'hg pull'). This also includes the fix for the embedded unarr build.