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