install qtbase5-dev (if you want to be able to remove that and its other dependencies after building, you may want to also explicitly install libqt5core5t64, libqt5gui5t64, libqt5widgets5t64 and libqt5dbus5t64 (instead of letting them being pulled by qtbase5-dev).
Thank you guys!
This was, till today, never required, since it was already there. Now, with a fresh install, these stuff were missing. My fault. Thanks alot for this! Gettin' old here .
# find out where one of those is:
find /usr -type f -name 'Qt5Core.pc'
# note the directory - the rest of them should all be there
# set the path to satisfy pkg-config. In my case:
export PKG_CONFIG_PATH=/usr/lib/x86_64-linux-gnu/pkgconfig
ah yeah, had that exact one on a mate's machine... apt said it was all installed but pkg-config just wasnt seeing it that PKG_CONFIG_PATH fix is the right shout, worth stickin in your .bashrc if your on a distro that likes movin them pkgconfig dirs about. definately a Debian/Trixie quirk this, never used to be a problem.
On my Debian 13 Xfce
All i do is install build-essential pkg-config libc6-dev libssl-dev libexpat1-dev libavcodec-dev libgl1-mesa-dev qtbase5-dev zlib1g-dev
Then ./configure, make, make install etc... works without problems.