Once cmake finds the proper Qt, it uses that Qt's qmake to query for the qtlib directory. It would be interesting to know what the output of the following command is on your problematic system:
Code:
qmake -query QT_INSTALL_LIBS
The path it returns is what QTLIB_DIR in the /usr/local/bin/sigil launch script should be populated with (if Sigil was built using the default system Qt5 and not a different version downloaded specifically for building Sigil).