Add support of embedded ft-index in zim for kiwix-serve and kiwix-search.

- kiwix-search will try the embedded ft-index if no indexPath is specified
  in the library.xml or command argument.
- kiwix-search also uses xapianSearcher and xapianSearcher needs zimlib,
  so add zimlib in compilation flag for searcher.
- kiwix-serve already depends on zimlib.

src/server/kiwix-serve.cpp

@@ -602,8 +602,12 @@ int main(int argc, char **argv) {
 	string humanReadableId = currentBook.getHumanReadableIdFromPath();
 	readers[humanReadableId] = reader;
 
-	/* Instanciate the ZIM index (if necessary) */
-	if (!indexPath.empty()) {
+	/* Try to instanciate the zim index.
+	 * If there is no specified indexPath, try to open the
+	 * embedded index in the zim. */
+	if (indexPath.empty()) {
+	    indexPath = zimPath;
+	}
 	try {
 	  kiwix::Searcher *searcher = new kiwix::XapianSearcher(indexPath);
 	  searcher->setProtocolPrefix("/");
@@ -616,7 +620,6 @@ int main(int argc, char **argv) {
       }
     }
   }
-  }
 
   /* Compute the Welcome HTML */
   string welcomeBooksHtml;