Autogen: Compile `mocs_compilation.cpp` first
mocs_compilation.cpp easily takes a long time to compile when it
moc files. When it was appended like before
we ran into the situation that all smaller sources were already compiled when
mocs_compilation.cpp got compiled at last. In that case a single core was
busy but all remaining cores stayed idle.
To optimize CPU core utilization we now prepend `mocs_compilation.cpp` to the sources list instead of appending it. This allows smaller source files to get compiled *while* the long lasting `mocs_compilation.cpp` gets compiled.