Initialize debug-file-directory from NIX_DEBUG_INFO_DIRS, a colon-separated list
of directories with separate debugging information files.

--- a/gdb/main.c
+++ b/gdb/main.c
@@ -551,3 +551,6 @@ captured_main_1 (struct captured_main_args *context)
 
-  debug_file_directory = relocate_gdb_directory (DEBUGDIR,
+  debug_file_directory = getenv("NIX_DEBUG_INFO_DIRS");
+
+  if (debug_file_directory == NULL)
+    debug_file_directory = relocate_gdb_directory (DEBUGDIR,
 						 DEBUGDIR_RELOCATABLE);