Considering this is an issue with LightDM do not find Qmake (just my guess), I do not have any idea about fix it and considering all other qt5 based ports builds fine, this is a LightDM issue. Then, before maintainer look in to it, I can be glad if someone using this lightweight display manager can suggest a workaround for have it compiled and installed.
Code:
moc --qt=5 QLightDM/greeter.h -o greeter_moc5.cpp
moc --qt=5 QLightDM/power.h -o power_moc5.cpp
moc: could not find a Qt installation of '5'
moc: could not find a Qt installation of '5'
gmake[4]: *** [Makefile:1020: greeter_moc5.cpp] Error 1
gmake[4]: *** Waiting for unfinished jobs....
gmake[4]: *** [Makefile:1020: power_moc5.cpp] Error 1
gmake[4]: Leaving directory '/usr/ports/x11/lightdm/work/lightdm-1.28.0/liblightdm-qt'
gmake[3]: *** [Makefile:484: all-recursive] Error 1