Since we use glog, we need to still rely on gFlags for as long as Abseil
logging is not yet released. To do this, we have the util/flag.h header in
SAPI, faking the newer style API of Abseil's flags library.
The rationale behind this is that glog includes/builds with gflags for
command line flag parsing and having more than one flag parsing library
active at the same time is asking for trouble.
PiperOrigin-RevId: 256982667
Change-Id: I9c5406cb9ef4458daebfec0b0713f9bddbf5c3ad
c7f7666d9d