Another day in Qt/CMAKE paradise. After having downloaded the sources for LMMS (instead of getting them from the Git repository) and opening the CMakeLists.txt file, which is a large buildscript, I had a little bit more succes.
Unfortunately, even though the compiler and the platform are detected correctly, CMAKE is unable to find libsndfile1:

CMake Error at CMakeLists.txt:119 (MESSAGE): LMMS requires libsndfile1 and libsndfile1-dev >= 1.0.11 - please install, remove CMakeCache.txt and try again!

Now, I have version 1.0.20 installed, and put the folder this library lives in, into the PATH variable (sic). Still, no luck >_<