ExternalProject: Only add git config setting with git 1.7.7 or later

1cb65e680d (ExternalProject: Prevent the noisy detached head
messages on checkout, 2021-01-17) unconditionally added the advice.detachedHead
git config setting, but it requires git 1.7.7 or later. Since it isn't fatal to not
have it, just noisier, only add it when it is supported.

Fixes: #22206

Modules/ExternalProject.cmake

@@ -2663,9 +2663,11 @@ function(_ep_add_download_command name)
     get_property(git_progress TARGET ${name} PROPERTY _EP_GIT_PROGRESS)
     get_property(git_config TARGET ${name} PROPERTY _EP_GIT_CONFIG)
 
-    # Make checkouts quiet when checking out a git hash (this avoids the
-    # very noisy detached head message)
-    list(PREPEND git_config advice.detachedHead=false)
+    # If git supports it, make checkouts quiet when checking out a git hash.
+    # This avoids the very noisy detached head message.
+    if(GIT_VERSION_STRING VERSION_GREATER_EQUAL 1.7.7)
+      list(PREPEND git_config advice.detachedHead=false)
+    endif()
 
     # For the download step, and the git clone operation, only the repository
     # should be recorded in a configured RepositoryInfo file. If the repo