Store the what-is-snac URL as a CPP constant.

html.c

@@ -988,9 +988,11 @@ d_char *html_user_footer(snac *snac, d_char *s)
     xs *s1 = xs_fmt(
         "<div class=\"snac-footer\">\n"
         "<a href=\"%s\">%s</a> - "
-        "powered by <abbr title=\"Social Networks Are Crap\">snac</abbr></div>\n",
+        "powered by <a href=\"%s\""
+        "<abbr title=\"Social Networks Are Crap\">snac</abbr></div></a>\n",
         srv_baseurl,
-        L("about this site")
+        L("about this site"),
+        WHAT_IS_SNAC_URL
     );
 
     return xs_str_cat(s, s1);

mastoapi.c

@@ -1221,7 +1221,7 @@ int mastoapi_get_handler(const xs_dict *req, const char *q_path,
         ins = xs_dict_append(ins, "domain",      host);
         ins = xs_dict_append(ins, "title",       host);
         ins = xs_dict_append(ins, "version",     "4.0.0 (not true; really " USER_AGENT ")");
-        ins = xs_dict_append(ins, "source_url",  "https:/" "/comam.es/what-is-snac");
+        ins = xs_dict_append(ins, "source_url",  WHAT_IS_SNAC_URL);
         ins = xs_dict_append(ins, "description", host);
 
         ins = xs_dict_append(ins, "short_description", host);

snac.h

@@ -5,6 +5,8 @@
 
 #define USER_AGENT "snac/" VERSION
 
+#define WHAT_IS_SNAC_URL "https:/" "/comam.es/what-is-snac"
+
 #define DIR_PERM 02770
 
 extern double disk_layout;