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.
I pulled ``` DYLD_LIBRARY_PATH ``` out and it still linked okay, so I'll pull that element out and use just flags if the latter is likely to cause less issues.