GCC 10+ use -fno-common by default, which means we need to declare this 'extern'.

There is no upstream to send this to.

--- libnih-1.0.3/nih-dbus-tool/output.h.old	2010-09-20 18:17:01.000000000 -0500
+++ libnih-1.0.3/nih-dbus-tool/output.h	2024-05-28 13:22:58.501847205 -0500
@@ -27,7 +27,7 @@
 
 NIH_BEGIN_EXTERN
 
-char *output_package;
+extern char *output_package;
 
 int   output          (const char *source_path, int source_fd,
 		       const char *header_path, int header_fd,