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

build/fbcode_builder/CMake/FindGflags.cmake

@@ -41,6 +41,16 @@ if (gflags_FOUND)
   # Re-export the config-specified libs with our local names
   set(LIBGFLAGS_LIBRARY ${gflags_LIBRARIES})
   set(LIBGFLAGS_INCLUDE_DIR ${gflags_INCLUDE_DIR})
+  if(NOT EXISTS "${gflags_INCLUDE_DIR}")
+    # The gflags-devel RPM on recent RedHat-based systems is somewhat broken.
+    # RedHat symlinks /lib64 to /usr/lib64, and this breaks some of the
+    # relative path computation performed in gflags-config.cmake.  The package
+    # config file ends up being found via /lib64, but the relative path
+    # computation it does only works if it was found in /usr/lib64.
+    # If gflags_INCLUDE_DIR does not actually exist, simply default it to
+    # /usr/include on these systems.
+    set(LIBGFLAGS_INCLUDE_DIR "/usr/include")
+  endif()
   set(LIBGFLAGS_FOUND ${gflags_FOUND})
   # cmake module compat
   set(GFLAGS_FOUND ${gflags_FOUND})
@@ -76,6 +86,20 @@ endif()
 # Compat with the gflags CONFIG based detection
 if (LIBGFLAGS_FOUND AND NOT TARGET gflags)
   add_library(gflags UNKNOWN IMPORTED)
-  set_target_properties(gflags PROPERTIES INTERFACE_INCLUDE_DIRECTORIES "${LIBGFLAGS_INCLUDE_DIR}")
-  set_target_properties(gflags PROPERTIES IMPORTED_LINK_INTERFACE_LANGUAGES "C" IMPORTED_LOCATION "${LIBGFLAGS_LIBRARY}")
+  if(TARGET gflags-shared)
+    # If the installed gflags CMake package config defines a gflags-shared
+    # target but not gflags, just make the gflags target that we define
+    # depend on the gflags-shared target.
+    target_link_libraries(gflags INTERFACE gflags-shared)
+    # Export LIBGFLAGS_LIBRARY as the gflags-shared target in this case.
+    set(LIBGFLAGS_LIBRARY gflags-shared)
+  else()
+    set_target_properties(
+      gflags
+      PROPERTIES
+        IMPORTED_LINK_INTERFACE_LANGUAGES "C"
+        IMPORTED_LOCATION "${LIBGFLAGS_LIBRARY}"
+        INTERFACE_INCLUDE_DIRECTORIES "${LIBGFLAGS_INCLUDE_DIR}"
+    )
+  endif()
 endif()