Look up .build-id files relative to the directories in the
colon-separated environment variable NIX_DEBUG_INFO_DIRS, in addition
to the existing debug-file-directory setting.

diff -ru --exclude '*gcore' --exclude '*pdtrace' gdb-8.0-orig/gdb/build-id.c gdb-8.0/gdb/build-id.c
--- gdb-8.0-orig/gdb/build-id.c	2017-06-04 17:51:26.000000000 +0200
+++ gdb-8.0/gdb/build-id.c	2017-07-28 13:18:10.797375927 +0200
@@ -67,8 +67,8 @@
 
 /* See build-id.h.  */
 
-gdb_bfd_ref_ptr
-build_id_to_debug_bfd (size_t build_id_len, const bfd_byte *build_id)
+static gdb_bfd_ref_ptr
+build_id_to_debug_bfd_in (const char *directories, size_t build_id_len, const bfd_byte *build_id)
 {
   char *link, *debugdir;
   VEC (char_ptr) *debugdir_vec;
@@ -78,7 +78,7 @@
   int alloc_len;
 
   /* DEBUG_FILE_DIRECTORY/.build-id/ab/cdef */
-  alloc_len = (strlen (debug_file_directory)
+  alloc_len = (strlen (directories)
 	       + (sizeof "/.build-id/" - 1) + 1
 	       + 2 * build_id_len + (sizeof ".debug" - 1) + 1);
   link = (char *) alloca (alloc_len);
@@ -86,7 +86,7 @@
   /* Keep backward compatibility so that DEBUG_FILE_DIRECTORY being "" will
      cause "/.build-id/..." lookups.  */
 
-  debugdir_vec = dirnames_to_char_ptr_vec (debug_file_directory);
+  debugdir_vec = dirnames_to_char_ptr_vec (directories);
   back_to = make_cleanup_free_char_ptr_vec (debugdir_vec);
 
   for (ix = 0; VEC_iterate (char_ptr, debugdir_vec, ix, debugdir); ++ix)
@@ -137,6 +137,30 @@
   return abfd;
 }
 
+gdb_bfd_ref_ptr
+build_id_to_debug_bfd (size_t build_id_len, const bfd_byte *build_id)
+{
+  gdb_bfd_ref_ptr abfd = build_id_to_debug_bfd_in(debug_file_directory, build_id_len, build_id);
+
+  if (abfd != NULL)
+    return abfd;
+
+  static int init = 0;
+  static char *env_var;
+  if (!init)
+    {
+      env_var = getenv("NIX_DEBUG_INFO_DIRS");
+      init = 1;
+    }
+
+  if (env_var)
+    {
+      abfd = build_id_to_debug_bfd_in(env_var, build_id_len, build_id);
+    }
+
+  return abfd;
+}
+
 /* See build-id.h.  */
 
 char *
diff -ru --exclude '*gcore' --exclude '*pdtrace' gdb-8.0-orig/gdb/symfile.c gdb-8.0/gdb/symfile.c
--- gdb-8.0-orig/gdb/symfile.c	2017-06-04 17:51:27.000000000 +0200
+++ gdb-8.0/gdb/symfile.c	2017-07-28 12:54:05.401586174 +0200
@@ -1415,8 +1415,8 @@
 			   struct cmd_list_element *c, const char *value)
 {
   fprintf_filtered (file,
-		    _("The directory where separate debug "
-		      "symbols are searched for is \"%s\".\n"),
+		    _("The directories where separate debug "
+		      "symbols are searched for are \"%s\".\n"),
 		    value);
 }