`mocs_compilation.cpp` easily takes a long time to compile when it
contains multiple `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.

Closes #18793