fix: Define both 'gflags' and 'gflags::gflags' alias targets

CMakeLists.txt

@@ -498,6 +498,10 @@ if (GFLAGS_IS_SUBPROJECT)
   foreach (type IN ITEMS static shared)
     foreach (opts IN ITEMS "_nothreads" "")
       if (TARGET gflags${opts}_${type})
+        # Define "gflags" alias for super-projects treating targets of this library as part of their own project
+        # (also for backwards compatibility with gflags 2.2.1 which only defined this alias)
+        add_library (gflags ALIAS gflags${opts}_${type})
+        # Define "gflags::gflags" alias for projects that support both find_package(gflags) and add_subdirectory(gflags)
         add_library (gflags::gflags ALIAS gflags${opts}_${type})
         break ()
       endif ()