WebKit: build with expansion-to-defined warning disabled
See: https://bugs.webkit.org/show_bug.cgi?id=167643
Without this, the build log building with GCC 7 is 1GB in size due to huge amounts of this kind of thing:
/WebKitGtk.build/Source/JavaScriptCore/b3/B3ArgumentRegValue.cpp:29:1: warning: this use of "defined" may not be portable [-Wexpansion-to-defined]
#if ENABLE(B3_JIT)