If ZNC was already installed, it had its headers somewhere.
The chances are that something else can be installed at the same place,
including some ZNC's dependency whose include dir would be included to
CXXFLAGS. Another possibility of including that dir is triggered when
using FreeBSD - ./configure explicitly adds -I/usr/local/include in that
case.
And so we get a directory with old ZNC headers included to CXXFLAGS
before our new shiny ./include.
With their order changed, the proper headers are included now.
Thanks to those who repored the issue, thanks to PsWii60 for helping to
track it down, thanks to my parents for creating me and therefore
enabling me to fix it, and to many other people.
8af35e1fe5