Install include files to a subdir by default

[KitsuneRal]

A similar change has been proposed in #270 but it hardcoded the directory name. Meanwhile the general need for such default is still around at least on Linux. And distributions will want to customise the name according to their naming conventions.

[noc0lour]

I already stumbled upon failing builds which included the "wrong" util.h.

[KitsuneRal]

A complete fix is pending on https://gitlab.com/b0/libqtolm/-/merge_requests/6