MDEV-12310 openat(<directory>, ...O_EXEC) fails on Illumos / Solaris

it could be
* O_SEARCH on Illumos
* O_EXEC on FreeBSD
* O_PATH on Linux
ugh

mysys/mysys_priv.h

@@ -89,9 +89,13 @@ void sf_free(void *ptr);
 
 void my_error_unregister_all(void);
 
-#if !defined(O_PATH) && defined(O_EXEC) /* FreeBSD */
+#ifndef O_PATH        /* not Linux */
+#if defined(O_SEARCH) /* Illumos */
+#define O_PATH O_SEARCH
+#elif defined(O_EXEC) /* FreeBSD */
 #define O_PATH O_EXEC
 #endif
+#endif
 
 #ifdef O_PATH
 #define HAVE_OPEN_PARENT_DIR_NOSYMLINKS