For me since importing Ladybird if I do:
./Meta/serenity.sh run lagom ladybird then ./Meta/serenity.sh run, the serenity build will fail (during the lagom step) after failing to link pthreads:

[0/5] Performing build step for 'lagom'
[0/2] Re-checking globbed directories...
[423/435] Linking CXX executable Userland/Shell/Shell
FAILED: Userland/Shell/Shell 
: && /usr/bin/clang++   Userland/Shell/CMakeFiles/Shell.dir/main.cpp.o -o Userland/Shell/Shell  -Wl,-rpath,/home/macdue/git/serenity/Build/lagom/Userland/Shell:/home/macdue/git/serenity/Build/lagom/Userland/Libraries/LibLine:/home/macdue/git/serenity/Build/lagom/Userland/Libraries/LibSyntax:/home/macdue/git/serenity/Build/lagom/Userland/Libraries/LibRegex:/home/macdue/git/serenity/Build/lagom/Userland/Libraries/LibUnicode:/home/macdue/git/serenity/Build/lagom/Userland/Libraries/LibCore  Userland/Shell/liblagom-shell.so.0.0.0  Userland/Libraries/LibMain/liblagom-main.a  Userland/Libraries/LibLine/liblagom-line.so.0.0.0  Userland/Libraries/LibCore/liblagom-core.so.0.0.0  -Wl,-rpath-link,/home/macdue/git/serenity/Build/lagom/Userland/Libraries/LibSyntax:/home/macdue/git/serenity/Build/lagom/Userland/Libraries/LibRegex:/home/macdue/git/serenity/Build/lagom/Userland/Libraries/LibUnicode && :
/usr/bin/ld: Userland/Shell/CMakeFiles/Shell.dir/main.cpp.o: undefined reference to symbol 'pthread_sigmask@@GLIBC_2.2.5'
/usr/bin/ld: /lib/x86_64-linux-gnu/libpthread.so.0: error adding symbols: DSO missing from command line
clang: error: linker command failed with exit code 1 (use -v to see invocation)
[430/435] Building CXX object Userland/Services/WebSocket/CMakeFiles/WebSocketServer.dir/ConnectionFromClient.cpp.o
ninja: build stopped: subcommand failed.
FAILED: lagom-prefix/src/lagom-stamp/lagom-build /home/macdue/git/serenity/Build/superbuild-x86_64/lagom-prefix/src/lagom-stamp/lagom-build 
cd /home/macdue/git/serenity/Build/lagom && /home/macdue/git/serenity/Toolchain/Local/cmake/bin/cmake --build .
ninja: build stopped: subcommand failed.

It's not super clear why this happens (and it does not happen for everyone, just me as a far as I know right now).
This is fixed by #16833, but there's probably a better fix out there.