-Wextra enables the new -Wexpansion-to-defined which causes OBS to run into

qemu-system-x86_64: terminating on signal 15 from pid 23592

Logfile got too big, killed job.

because of a huge swat of warnings from code like

[ 5443s] ../WTF/wtf/Atomics.h:221:1: warning: this use of "defined" may not be
portable [-Wexpansion-to-defined]
[ 5443s]  #if OS(WINDOWS) && !COMPILER(GCC)
[ 5443s]  ^~~~~~~~~~~~~~~~

basically the warning says that sth like

#define OS(x) defined(x)
#if OS(WINDOWS)

is not something that is allowed by the language (but works in GCC).

I suggest to add -Wno-expansion-to-defined.