options: add a deprecation warning printing mechanism

We have a warning mechanism for removed and for replaced options, but
none yet for options which have been simply deprecated.

For the following commit.

(Fun fact: just adding the m_option field increases binary size by
14KB.)

1 files changed, 4 insertions, 0 deletions

diff --git a/options/m_option.h b/options/m_option.h
index 4066a36d81..e77452af69 100644
--- a/options/m_option.h
+++ b/options/m_option.h
@@ -333,6 +333,10 @@ struct m_option {
 
     // Initialize variable to given default before parsing options
     const void *defval;
+
+    // Print a warning when this option is used (for options with no direct
+    // replacement.)
+    const char *deprecation_message;
 };