https://bugs.gentoo.org/787116
https://github.com/libretro/snes9x/commit/da1fc9a69d8fc776f1b13b080bc27cf329c52a1f

From da1fc9a69d8fc776f1b13b080bc27cf329c52a1f Mon Sep 17 00:00:00 2001
From: Alberto Fustinoni <alberto@aftnet.net>
Date: Thu, 10 Sep 2020 22:22:46 +0900
Subject: [PATCH] Build fix

---
 conffile.cpp | 2 +-
 conffile.h   | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/conffile.cpp b/conffile.cpp
index 817afae7f..5e121e2cc 100644
--- a/conffile.cpp
+++ b/conffile.cpp
@@ -452,7 +452,7 @@ void ConfigFile::ClearLines()
     }
 }
 
-bool ConfigFile::ConfigEntry::section_then_key_less::operator()(const ConfigEntry &a, const ConfigEntry &b) {
+bool ConfigFile::ConfigEntry::section_then_key_less::operator()(const ConfigEntry &a, const ConfigEntry &b) const {
 	if(curConfigFile && a.section!=b.section){
 		const int sva = curConfigFile->GetSectionSize(a.section);
 		const int svb = curConfigFile->GetSectionSize(b.section);
diff --git a/conffile.h b/conffile.h
index ba69812df..c09cea362 100644
--- a/conffile.h
+++ b/conffile.h
@@ -90,18 +90,18 @@ class ConfigFile {
 		mutable bool used;
 
         struct section_then_key_less {
-            bool operator()(const ConfigEntry &a, const ConfigEntry &b);
+            bool operator()(const ConfigEntry &a, const ConfigEntry &b) const;
         };
 
         struct key_less {
-            bool operator()(const ConfigEntry &a, const ConfigEntry &b) const{
+            bool operator()(const ConfigEntry &a, const ConfigEntry &b) const {
                 if(a.section!=b.section) return a.section<b.section;
                 return a.key<b.key;
             }
         };
 
         struct line_less {
-            bool operator()(const ConfigEntry &a, const ConfigEntry &b){
+            bool operator()(const ConfigEntry &a, const ConfigEntry &b) const {
 				if(a.line==b.line) return (b.val.empty() && !a.val.empty()) || a.key<b.key;
                 if(b.line<0) return true;
                 if(a.line<0) return false;