add werror flags when making a debug build using llvm (#4344)

* add werror flags when making a debug build using llvm

this would get bugs like
https://github.com/Cockatrice/Cockatrice/pull/4337
get signalled earlier to us

* fix error: 'Servatrice_DatabaseInterface::registerUser' hides overloaded virtual function

* remove unused field

* mac machines have 3 cores

see
https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources

* typo

.github/workflows/ci-builds.yml

@@ -271,6 +271,8 @@ jobs:
 
       - name: Build on Xcode ${{matrix.xcode}}
         shell: bash
+        env:
+          CMAKE_BUILD_PARALLEL_LEVEL: 3  # mac machines actually have 3 cores
         run: .ci/compile.sh ${{matrix.type}} --server
 
       - name: Test