make it compile with -O2 -Werror

src/helpers.cpp

@@ -18,6 +18,7 @@
  *   51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA.              *
  ***************************************************************************/
 
+#include <cassert>
 #include <cstring>
 #include <algorithm>
 #include <iostream>
@@ -298,7 +299,7 @@ int CaseInsensitiveStringComparison::operator()(const std::string &a, const std:
 
 bool CaseInsensitiveSorting::operator()(const MPD::Item &a, const MPD::Item &b)
 {
-	bool result;
+	bool result = false;
 	if (a.type == b.type)
 	{
 		switch (a.type)
@@ -323,6 +324,8 @@ bool CaseInsensitiveSorting::operator()(const MPD::Item &a, const MPD::Item &b)
 						break;
 				}
 				break;
+			default: // there is no other option, silence compiler
+				assert(false);
 		}
 	}
 	else

src/lyrics.cpp

@@ -375,9 +375,10 @@ void Lyrics::Edit()
 	
 	ShowMessage("Opening lyrics in external editor...");
 	
+	GNUC_UNUSED int res;
 	if (Config.use_console_editor)
 	{
-		system(("/bin/sh -c \"" + Config.external_editor + " \\\"" + itsFilename + "\\\"\"").c_str());
+		res = system(("/bin/sh -c \"" + Config.external_editor + " \\\"" + itsFilename + "\\\"\"").c_str());
 		// below is needed as screen gets cleared, but apparently
 		// ncurses doesn't know about it, so we need to reload main screen
 		endwin();
@@ -385,7 +386,7 @@ void Lyrics::Edit()
 		curs_set(0);
 	}
 	else
-		system(("nohup " + Config.external_editor + " \"" + itsFilename + "\" > /dev/null 2>&1 &").c_str());
+		res = system(("nohup " + Config.external_editor + " \"" + itsFilename + "\" > /dev/null 2>&1 &").c_str());
 }
 
 #ifdef HAVE_CURL_CURL_H

src/status.cpp

@@ -386,8 +386,9 @@ void NcmpcppStatusChanged(MPD::Connection *, MPD::StatusChanges changed, void *)
 	{
 		if (myPlaylist->isPlaying())
 		{
+			GNUC_UNUSED int res;
 			if (!Config.execute_on_song_change.empty())
-				system(Config.execute_on_song_change.c_str());
+				res = system(Config.execute_on_song_change.c_str());
 			
 #			ifdef HAVE_CURL_CURL_H
 			if (Config.fetch_lyrics_in_background)