update FindGflags.cmake to work on CentOS 8.x (#1409)
update FindGflags.cmake to work on CentOS 8.x (#1409)

Summary:
Update FindGflags.cmake to work on recent CentOS and RedHat distributions.

On these distributions the CMake package configuration installed by the
gflags-devel RPM is slightly broken, and sets `gflags_INCLUDE_DIR` to a
directory that does not exist.  This happens because `/lib64` is symlinked to
`/usr/lib64`, and CMake ends up searching `/lib64` before `/usr/lib64` by
default when searching for packages.  Therefore it finds `gflags-config.cmake`
via the `/lib64` symlink.  However, `gflags-config.cmake` computes the include
directory relative to where its config file was found, and this relative path
computation only works when using the actual `/usr/lib64` path where it was
installed.  When found via `/lib64` it returns a bogus `//include` path that
does not exist.

This updates `FindGflags.cmake` to verify if the `gflags_INCLUDE_DIR` path
actually exists, and set it to a sane location instead.

This also updates the code to use the `gflags-shared` target that is exported
by the default gflags package configuration if it was only built as a shared
library.

Pull Request resolved: https://github.com/facebook/folly/pull/1409

Reviewed By: yfeldblum

Differential Revision: D23588288

fbshipit-source-id: b68a717953ae0521f568d7bcfd05ca33cd6dc578

facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro