misc: Allow to install shared lib gflags on centos9 (#11889)

Summary:
This pr just reverts one line of change by #10732. I think it is not necessary to fully align gflags build with velox build. It means we should set `BUILD_SHARED_LIBS=ON` for gflags when `VELOX_BUILD_SHARED=ON`, but it is not necessary to set `BUILD_SHARED_LIBS=OFF` for gflags when `VELOX_BUILD_SHARED=OFF`. We can just make the shared lib always be produced, as I also note `setup-ubuntu.sh` always installs shared gflags from package (see [link](https://github.com/facebookincubator/velox/blob/main/scripts/setup-ubuntu.sh#L120)). Another reason for this fix is, it is better to keep using same lib type (static or shared) for gflags and glog to avoid any potential issues.

Related discussion on this issue: #10732 (comment)

Pull Request resolved: #11889

Reviewed By: xiaoxmeng

Differential Revision: D67414560

Pulled By: pedroerp

fbshipit-source-id: a797f44c2b659ca680ac555928a9f9d64ac9c8f0

scripts/setup-centos9.sh

@@ -90,7 +90,7 @@ function install_gflags {
   # Remove an older version if present.
   dnf remove -y gflags
   wget_and_untar https://github.com/gflags/gflags/archive/v2.2.2.tar.gz gflags
-  cmake_install_dir gflags -DBUILD_SHARED_LIBS="$VELOX_BUILD_SHARED" -DBUILD_STATIC_LIBS=ON -DBUILD_gflags_LIB=ON -DLIB_SUFFIX=64
+  cmake_install_dir gflags -DBUILD_SHARED_LIBS=ON -DBUILD_STATIC_LIBS=ON -DBUILD_gflags_LIB=ON -DLIB_SUFFIX=64
 }
 
 function install_glog {