Store the crawl version in the textdatabases in SAVEDIR and
upgrade the databases when the crawl version changes.

By default crawl checks for a mtime difference on files in DATADIR to see if an
upgrade is required, but guix nulls all file dates,
and crawl would never upgrade saves.

diff -ur a/source/database.cc b/source/database.cc
--- a/source/database.cc	2018-08-09 21:49:26.000000000 -0400
+++ b/source/database.cc	2018-10-07 18:06:41.022445789 -0400
@@ -24,6 +24,7 @@
 #include "stringutil.h"
 #include "syscalls.h"
 #include "unicode.h"
+#include "version.h"
 
 // TextDB handles dependency checking the db vs text files, creating the
 // db, loading, and destroying the DB.
@@ -54,6 +55,7 @@
     vector<string> _input_files;
     DBM* _db;
     string timestamp;
+    string version;
     TextDB *_parent;
     const char* lang() { return _parent ? Options.lang_name : 0; }
 public:
@@ -163,7 +165,7 @@
 
 TextDB::TextDB(const char* db_name, const char* dir, vector<string> files)
     : _db_name(db_name), _directory(dir), _input_files(files),
-      _db(nullptr), timestamp(""), _parent(0), translation(0)
+      _db(nullptr), timestamp(""), version(""),  _parent(0), translation(0)
 {
 }
 
@@ -171,7 +173,7 @@
     : _db_name(parent->_db_name),
       _directory(parent->_directory + Options.lang_name + "/"),
       _input_files(parent->_input_files), // FIXME: pointless copy
-      _db(nullptr), timestamp(""), _parent(parent), translation(nullptr)
+      _db(nullptr), timestamp(""), version(""), _parent(parent), translation(nullptr)
 {
 }
 
@@ -186,6 +188,9 @@
         return false;
 
     timestamp = _query_database(*this, "TIMESTAMP", false, false, true);
+    version = _query_database(*this, "VERSION", false, false, true);
+    if (version.empty())
+        return false;
     if (timestamp.empty())
         return false;
 
@@ -229,6 +234,9 @@
     string ts;
     bool no_files = true;
 
+    if (string(Version::Long) != version)
+        return true;
+
     for (const string &file : _input_files)
     {
         string full_input_path = _directory + file;
@@ -246,7 +254,7 @@
         ts += buf;
     }
 
-    if (no_files)
+    if (no_files && version.empty())
     {
         // No point in empty databases, although for simplicity keep ones
         // for disappeared translations for now.
@@ -312,7 +320,10 @@
             _store_text_db(full_input_path, _db);
         }
     }
+
+    string current_version = string(Version::Long);
     _add_entry(_db, "TIMESTAMP", ts);
+    _add_entry(_db, "VERSION", current_version);
 
     dbm_close(_db);
     _db = 0;
200'>2017-07-03</span></td><td><a href='/guix/commit/tests/discovery.scm?id=960c6ce96d746cf19829ad26e092ec5dad2a5c62'>discovery: Recurse into directories pointed to by a symlink.</a><span class='msg-avail'>...<span class='msg-tooltip'>Reported by Christopher Baines &lt;mail@cbaines.net&gt;
and Alex Kost &lt;alezost@gmail.com&gt;
at &lt;https://lists.gnu.org/archive/html/guix-devel/2017-06/msg00290.html&gt;.

* guix/discovery.scm (scheme-files): When ENTRY is a symlink that
doesn't end in '.scm', call 'stat' and recurse if it points to a
directory.
* tests/discovery.scm ("scheme-modules recurses in symlinks to
directories"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-06-18 00:14:07 +0200'>2017-06-18</span></td><td><a href='/guix/commit/tests/discovery.scm?id=d46c4423f46278bd2f96770ceb0667431414349e'>discovery: 'scheme-files' returns '() for a non-accessible directory.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes a regression introduced in
d27cc3bfaafe6b5b0831e88afb1c46311d382a0b.

Reported by Ricardo Wurmus &lt;rekado@elephly.net&gt;.

* guix/discovery.scm (scheme-files): Catch 'scandir*' system errors.
Return '() and optionally raise a warning upon 'system-error'.
* tests/discovery.scm ("scheme-modules, non-existent directory"): New
test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-05-03 23:50:15 +0200'>2017-05-03</span></td><td><a href='/guix/commit/tests/discovery.scm?id=cd903ef7871170d3c4eced45418459d293ef48a7'>Add (guix discovery).</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/discovery.scm, tests/discovery.scm: New files.
* gnu/packages.scm (scheme-files, file-name-&gt;module-name)
(scheme-modules, all-package-modules): Remove.
(fold-packages): Rewrite in terms of 'fold-module-public-variables'.
* gnu/tests.scm: Use (guix discovery).
* Makefile.am (MODULES): Add guix/discovery.scm.
(SCM_TESTS): Add tests/discovery.scm.
</span></span></td><td>Ludovic Courtès</td></tr>