Actually I'm only using Qt5 for building sigil, the default Qt is still the system version (4.8 I think). I only set the PATH for the session doing the build. Once it picks up the correct qmake cmake configures everything else.
I figured I was going to have to set LD_LIBRARY_PATH but found it worked even if I didn't and have found that the executable gets compiled with the full path to the library in it. If that path exists it uses it, otherwise it checks LD_LIBRARY_PATH. (I've seen this before and I can't remember if it's a particular compile flag but I don't think it does it by default.)
Of course setting LD_LIBRARY_PATH would be neater and 'safer' but it's handy not to have to.