glog: gflags compatibility fix

Closes #34416. The upstream issue tracker for Glog is pretty dead, and gflags development seems to be outpacing it, so vendoring the old version of gflags is a workaround.
Which part? The ``` DYLD_LIBRARY_PATH ``` is recommended by the ``` gflags ``` configure script itself, and both the ``` LDFLAGS ``` and ``` cflags ``` are required to correctly hook into the internal ``` gflags ```. I can try to remove the ``` DYLD_LIBRARY_PATH ``` and see what happens though if that's the element you're worried about causing issues?