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
@@ -25,6 +25,7 @@
 #include "syscalls.h"
 #include "threads.h"
 #include "unicode.h"
+#include "version.h"
 
 // TextDB handles dependency checking the db vs text files, creating the
 // db, loading, and destroying the DB.
@@ -55,6 +56,7 @@
     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;
@@ -245,7 +253,7 @@
         ts += buf;
     }
 
-    if (no_files && timestamp.empty())
+    if (no_files && timestamp.empty() && version.empty())
     {
         // No point in empty databases, although for simplicity keep ones
         // for disappeared translations for now.
@@ -313,7 +321,10 @@
             _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;
id kill -1 bug on the Hurd.</a><span class='msg-avail'>...</span></td><td>Manolis Ragkousis</td></tr>
<tr><td><span title='2020-01-12 00:19:09 +0100'>2020-01-12</span></td><td><a href='/guix/commit/nix/libutil?id=7033c7692ccbbbad8f7b9952015de071a5588e87'>daemon: Account for deleted store files when deduplication is on.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-11-29 15:54:20 +0100'>2019-11-29</span></td><td><a href='/guix/commit/nix/libutil?id=b6b014bf42f68fcb71d37204328babb81e0e0855'>daemon: 'pathExists' uses 'statx' when available.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-11-27 00:03:03 +0100'>2019-11-27</span></td><td><a href='/guix/commit/nix/libutil?id=fbecb5cddb2d36cc1a29dac3751b017fa531383c'>daemon: 'deletePath' uses 'statx' when available.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-08-30 01:33:45 +0200'>2019-08-30</span></td><td><a href='/guix/commit/nix/libutil?id=546a709f202024c8a5173ad372a87ddc1c284c63'>daemon: Don't reply on 'st_blocks'.</a><span class='msg-avail'>...</span></td><td>Eelco Dolstra</td></tr>