http://lists.schmorp.de/pipermail/rxvt-unicode/2023q2/002652.html

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -95,7 +95,8 @@ check_function_exists(setreuid HAVE_SETREUID)
 check_function_exists(setresuid HAVE_SETRESUID)
 
 check_c_source_compiles(
-  "#include <stdlib.h>
+  "#define _XOPEN_SOURCE 500
+   #include <stdlib.h>
 
    int main ()
    {