Add ${LIBGFLAGS_LIBRARY} to IMPORTED_LINK_INTERFACE_LIBRARIES of glog::glog (#9169)
Add ${LIBGFLAGS_LIBRARY} to IMPORTED_LINK_INTERFACE_LIBRARIES of glog::glog (#9169)

Summary:
X-link: https://github.com/facebook/hhvm/pull/9169

Pull Request resolved: https://github.com/facebook/wangle/pull/208

X-link: https://github.com/facebookincubator/reindeer/pull/4

X-link: https://github.com/facebookincubator/velox/pull/2445

X-link: https://github.com/facebookincubator/hsthrift/pull/98

X-link: https://github.com/facebookincubator/katran/pull/171

X-link: https://github.com/facebookincubator/mvfst/pull/272

X-link: https://github.com/facebook/fboss/pull/118

X-link: https://github.com/facebookincubator/fizz/pull/81

X-link: https://github.com/facebookexperimental/eden/pull/127

X-link: https://github.com/facebook/sapling-staging/pull/7

X-link: https://github.com/facebookexperimental/edencommon/pull/4

X-link: https://github.com/facebookexperimental/rust-shed/pull/34

X-link: https://github.com/facebook/fbthrift/pull/521

X-link: https://github.com/facebook/watchman/pull/1055

X-link: https://github.com/facebook/folly/pull/1854

X-link: https://github.com/facebook/openr/pull/140

X-link: https://github.com/facebook/fb303/pull/31

Previously Folly provided a polyfill of the `GflagsFlagSaver` symbol, which was removed since https://github.com/facebook/folly/commit/4dadde1256f74e756510a499c86459967f78afb9. Therefore, `glog` should solve the `GflagsFlagSaver` symbol directly from `gflags`.

This diff added `gflags` libraries as a dependency of the CMake project `glog::glog` into `fbcode_builder`'s `FindGlog.cmake`, so that the `fbcode_builder` users will be able to automatically link with `gflags`.

Reviewed By: alexeyt

Differential Revision: D39220438

fbshipit-source-id: 485d67bfeace955f83a4961372680f12eb9cc1a5

facebook/wangle,facebook/wangle,facebook/wangle