SDL: cmake: fixed iconv detection test program

SDL Commits
1 
From d486de634917b80c53362d0f62a9d1f762c8f4fb Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Sun, 26 Nov 2023 01:55:28 +0300
Subject: [PATCH] cmake: fixed iconv detection test program

Fixes https://github.com/libsdl-org/SDL/issues/8614
---
 CMakeLists.txt | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 52e3f900591c..91b660e52c04 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1105,7 +1105,7 @@ if(SDL_LIBC)
         #include <stddef.h>
         #include <iconv.h>
         int main(int argc, char **argv) {
-            return iconv_open(NULL,NULL);
+            return !iconv_open(NULL,NULL);
         }" ICONV_IN_LIBC)
 
       cmake_push_check_state()
@@ -1114,7 +1114,7 @@ if(SDL_LIBC)
         #include <stddef.h>
         #include <iconv.h>
         int main(int argc, char **argv) {
-            return iconv_open(NULL,NULL);
+            return !iconv_open(NULL,NULL);
         }" ICONV_IN_LIBICONV)
       cmake_pop_check_state()