webkit-gtk: Merge changes from trunk

webkit-gtk: Get farther towards building on Tiger (#37828)
webkit-gtk: Also set __MAC_OS_X_VERSION_MAX_ALLOWED for Tiger, since it's not provided by Availability.h
webkit-gtk: don't backtrace() on Tiger (#37882)

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>