Posted by LordOfBikes on
URL: https://forum.librecad.org/Build-on-Mint-18-tp5716522p5716536.html

The environment was the same for me after a fresh Mint setup.
Seems Mint is using Qt4 as default.
To change this type

export QT_SELECT=qt5

This is only temporary, here are some hints how to set this permanent:
https://unix.stackexchange.com/questions/116254/how-do-i-change-which-version-of-qt-is-used-for-qmake

qmake -v must run without the whole path and should show a Qt5 version.
When you run Qt5 qmake with absolute path in this environment, it will create Makefiles with Qt4 settings, as this is the system default. These Makefiles will very likely produce the error from your first post.

Mint 18.3 should install Qt5.5 with apt, which is sufficient for LibreCAD. I don't know, if qtchooser recognizes your manual installed Qt5.10. You can check this with

qtchooser -list-versions

Anyway there seems to be something messed up with apt. As I said, on my fresh Mint, apt installed all packages including dependencies.
You can try to fix this with

sudo apt-get -f install

If this doesn't work, think back, if you have modified Mints package source settings in past.

Armin