Fix Linux compilation: define _GNU_SOURCE for dladdr/Dl_info

On Linux, dlfcn.h only exposes dladdr and Dl_info when _GNU_SOURCE is
defined. macOS exposes them unconditionally, which is why local tests
passed but CI on ubuntu failed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gbaraldi

src/scripts/juliac-jl-options-shim.c

@@ -14,6 +14,10 @@
  * "juliac-jl-options-body.h".
  */
 
+#ifndef _WIN32
+#define _GNU_SOURCE
+#endif
+
 #include <julia.h>
 #include <stdlib.h>