devel/cbang: databases/postgresql-plv8js: Chase v8 upgrade

V8 15.4 headers refuse to compile ("Non-CppGC MicrotaskQueue definition
is not supported") unless the embedder defines V8_CPPGC_MICROTASK_QUEUE,
matching how lang/v8 itself is built.

databases/postgresql-plv8js: pass the define in CXXFLAGS.

devel/cbang: define it in the v8 scons config before the v8.h header
check, which otherwise fails and silently disables V8 support, breaking
packaging. Also record it in cbang/config.h for consumers.