lib: fix clang build

Long story short, fix build on recent (3.2+) clang.

The long story for posterity follows.

gcc 4.6 added new warnings about structs with greater visibility than
their fields. The warnings were silenced by adjusting visibility in

commit d5523ead90
Author: Carl Worth <cworth@cworth.org>
Date:   Wed May 11 13:23:13 2011 -0700

    Mark some structures in the library interface with visibility=default attribute.

Later on,

commit 3b76adf9e2
Author: Austin Clements <amdragon@MIT.EDU>
Date:   Sat Jan 14 19:17:33 2012 -0500

    lib: Add support for automatically excluding tags from queries

changed visibility of struct _notmuch_string_list for the same reason, and

commit 1a53f9f116
Author: Mark Walters <markwalters1009@gmail.com>
Date:   Thu Mar 1 22:30:38 2012 +0000

    lib: Add the exclude flag to notmuch_query_search_threads

split the struct _notmuch_string_list and its typedef
notmuch_string_list_t as a way to make a forward declaration for
_notmuch_thread_create().

The subtle difference was that the struct definition now had 'visible'
in it, while the typedef didn't, and it was within the #pragma GCC
visibility push(hidden) block. This went unnoticed, as the then common
versions of clang didn't care about this.

A later change in clang (I did not dig into when this change was
introduced) caused the following error:

CXX  -O2 lib/database.o
In file included from lib/database.cc:21:
In file included from ./lib/database-private.h:33:
./lib/notmuch-private.h:479:8: error: visibility does not match previous declaration
struct visible _notmuch_string_list {
       ^
./lib/notmuch-private.h:67:33: note: expanded from macro 'visible'
                                ^
./lib/notmuch-private.h:52:13: note: previous attribute is here
            ^
1 error generated.
make: *** [lib/database.o] Error 1

This is slightly misleading due to the reference to the #pragma. The
real culprit is the typedef within the #pragma.

We could just add 'visible' to the typedef, or move the typedef
outside of the #pragma, and be done with it, but juggle the
declarations a bit to accommodate moving the typedef back with the
struct, and keep the visibility attribute in one place.

The problem was originally reported by Simonas Kazlauskas
<s@kazlauskas.me> in id:20130418102507.GA23688@godbox but I was only
able to reproduce and investigate now that I upgraded clang.

lib/notmuch-private.h

@@ -162,8 +162,6 @@ typedef enum _notmuch_find_flags {
 
 typedef struct _notmuch_doc_id_set notmuch_doc_id_set_t;
 
-typedef struct _notmuch_string_list notmuch_string_list_t;
-
 /* database.cc */
 
 /* Lookup a prefix value by name.
@@ -228,17 +226,6 @@ _notmuch_directory_create (notmuch_database_t *notmuch,
 unsigned int
 _notmuch_directory_get_document_id (notmuch_directory_t *directory);
 
-/* thread.cc */
-
-notmuch_thread_t *
-_notmuch_thread_create (void *ctx,
-			notmuch_database_t *notmuch,
-			unsigned int seed_doc_id,
-			notmuch_doc_id_set_t *match_set,
-			notmuch_string_list_t *excluded_terms,
-			notmuch_exclude_t omit_exclude,
-			notmuch_sort_t sort);
-
 /* message.cc */
 
 notmuch_message_t *
@@ -476,11 +463,11 @@ typedef struct _notmuch_string_node {
     struct _notmuch_string_node *next;
 } notmuch_string_node_t;
 
-struct visible _notmuch_string_list {
+typedef struct visible _notmuch_string_list {
     int length;
     notmuch_string_node_t *head;
     notmuch_string_node_t **tail;
-};
+} notmuch_string_list_t;
 
 notmuch_string_list_t *
 _notmuch_string_list_create (const void *ctx);
@@ -509,6 +496,17 @@ notmuch_filenames_t *
 _notmuch_filenames_create (const void *ctx,
 			   notmuch_string_list_t *list);
 
+/* thread.cc */
+
+notmuch_thread_t *
+_notmuch_thread_create (void *ctx,
+			notmuch_database_t *notmuch,
+			unsigned int seed_doc_id,
+			notmuch_doc_id_set_t *match_set,
+			notmuch_string_list_t *excluded_terms,
+			notmuch_exclude_t omit_exclude,
+			notmuch_sort_t sort);
+
 NOTMUCH_END_DECLS
 
 #ifdef __cplusplus