I get everything to compile now. It's not in particular nice how I do it. I add the path of the sqlite3.h file and the sqlite3.dll manually in the qmake file and remove the dependency on the sqlite3 development package for windows. I don't have to make any other changes to source files. It also works to statically link the sqlite3 library by just including the full flat sqlite3.c file into the project, but I liked this even less. If I find a better way, I'll let you know.

unix: PKGCONFIG += sqlite3 
win32: CONFIG += sql

win32:LIBS += "C:/<path>/sqlite3.dll"
win32:INCLUDEPATH += "C:/<path>"    #path to sqlite3.h header file

Thank you for your help @Joshua and @scorpio810

I am able to compile qet now by commenting out the lines in the qmake file and it seems to work. I had to add the "sqlite3.h" header file to the Qt include directory. Otherwise I would get the following error

projectdatabase.cpp:33: error: sqlite3.h: No such file or directory
..\qelectrotech-source-mirror\sources\dataBase\projectdatabase.cpp:33:10: fatal error: sqlite3.h: No such file or directory
 #include <sqlite3.h>

But fortunately there weren't any further compile or link errors after that.

I wasn't so far able to compile it with sqlite3. It seems like qmake tries to look for the "libsqlite3-dev" package, which I cannot install for windows. But I am not that familiar with the Qt build process to figure out where to adapt this. @Joshua I was able to build the sqlite3.lib file with the instructions you linked. It's just not so clear how to make the build process digest it. I will investigate this a little bit further and will let you know if i find a way to do it.

I am trying to build QElectroTech from source on Windows. I think I am almost there. However, I am stuck when running qmake, I get the following error

12:58:22: Starting: "C:\Qt\5.15.0\mingw81_64\bin\qmake.exe" C:\Users\Lucas\Development_qet\qelectrotech-source-mirror\qelectrotech.pro -spec win32-g++ "CONFIG+=debug" "CONFIG+=qml_debug"
Project ERROR: sqlite3 development package not found
12:58:22: The process "C:\Qt\5.15.0\mingw81_64\bin\qmake.exe" exited with code 3.
Error while building/deploying project qelectrotech (kit: Desktop Qt 5.15.0 MinGW 64-bit)
When executing step "qmake"

Any idea how to fix this? This might of course also be a problem with my Qt installation.