sdl12-compat: Apparently g++'s preprocessor defines __unix__ but not unix. :/

SDL Commits
1 
From e20b7fc8fe7a17caeb360fd85b19c598bf0c32fb Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Tue, 22 Jun 2021 00:09:41 -0400
Subject: [PATCH] Apparently g++'s preprocessor defines __unix__ but not unix. 
 :/

Fixes lincity-ng failing to compile with the replacement headers because
iconv wasn't set up correctly.
---
 include/SDL/SDL_config.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/SDL/SDL_config.h b/include/SDL/SDL_config.h
index 1a71bc0..56cbbe5 100644
--- a/include/SDL/SDL_config.h
+++ b/include/SDL/SDL_config.h
@@ -72,7 +72,7 @@ stage, though. Send patches if your platform lacks something. */
 #define HAVE_MATH_H 1
 #endif
 
-#if defined(unix) || defined(__APPLE__)
+#if defined(unix) || defined(__unix__) || defined(__APPLE__)
 #define HAVE_ICONV_H 1
 #define HAVE_SIGNAL_H 1
 #endif
@@ -156,7 +156,7 @@ stage, though. Send patches if your platform lacks something. */
 #define HAVE_SEM_TIMEDWAIT 1
 #endif
 
-#if defined(unix) || defined(__APPLE__)
+#if defined(unix) || defined(__unix__) || defined(__APPLE__)
 #define HAVE_ICONV 1
 #define HAVE_SIGACTION 1
 #define HAVE_SA_SIGACTION 1