Hi, This is a little complicated because Amarok and Clementine both need liblastfm compiled with Qt4.
It looks like the latest version 1.1.0 (as yet untagged) allows for coinstallation of the -dev files as well. So it seems like the best way forward is to upgrade to that version, and then have that source package build both the Qt4 and Qt5 binaries. It's doable, but you'd need to do more by hand in debian/rules. It's been a while since this package mattered much to me. Would you be interested in taking over maintenance? John