CMake Error: The source directory "/home/alex/Applications/puredata/PureVST-0.5/build" does not appear to contain CMakeLists.txt.
You are in the "build" directory, but the CMakeLists.txt is one level above.
Perhaps something is missing in the command, like a double dot?
That's exactly it:
cmake -DCMAKE_BUILD_TYPE=Release ..
Christof