Fix compatibility with newer libconfig.

Patch copied from upstream source repository:

https://gitlab.com/lierolibre/lierolibre/commit/b27e3604aa6bfbfcc50db1000b394d06c87ae2f2

diff --git a/src/common.cpp b/src/common.cpp
index 2d6ada5..4942b05 100644
--- a/src/common.cpp
+++ b/src/common.cpp
@@ -162,7 +162,7 @@ void Texts::loadFromCFG(std::string cfgFilePath)
 	const libconfig::Setting &sgmodes = texts["gameModes"];
 	for(int i = 0; i < 4; ++i)
 	{
-		gameModes[i] = (char const*)sgmodes["gameModes" + to_string(i)];
+		gameModes[i] = (char const*)sgmodes[("gameModes" + to_string(i)).c_str()];
 	}
 
 	const libconfig::Setting &sgmspec = texts["gameModeSpec"];
@@ -181,13 +181,13 @@ void Texts::loadFromCFG(std::string cfgFilePath)
 	const libconfig::Setting &swstates = texts["weapStates"];
 	for(int i = 0; i < 3; ++i)
 	{
-		 weapStates[i] = (char const*)swstates["weapStates" + to_string(i)];
+		 weapStates[i] = (char const*)swstates[("weapStates" + to_string(i)).c_str()];
 	}
 
 	const libconfig::Setting &sknames = texts["keyNames"];
 	for(int i = 1; i < 177; ++i) // First key starts at 1
 	{
-		keyNames[i] = (char const*)sknames["keyNames" + to_string(i)];
+		keyNames[i] = (char const*)sknames[("keyNames" + to_string(i)).c_str()];
 	}
 
 	selWeap = (char const*)texts["selWeap"];
@@ -315,8 +315,8 @@ void Common::loadPaletteFromCFG(std::string cfgFilePath)
 	const libconfig::Setting &scanim = palette["colorAnim"];
 	for(int i = 0; i < 4; ++i)
 	{
-		colorAnim[i].from = (int)scanim["colorAnim" + to_string(i) + "from"];
-		colorAnim[i].to = (int)scanim["colorAnim" + to_string(i) + "to"];
+		colorAnim[i].from = (int)scanim[("colorAnim" + to_string(i) + "from").c_str()];
+		colorAnim[i].to = (int)scanim[("colorAnim" + to_string(i) + "to").c_str()];
 	}
 }
 
@@ -383,7 +383,7 @@ void Common::loadMaterialsFromCFG(std::string cfgFilePath)
 
 	for(int i = 0; i < 256; ++i)
 	{
-		const libconfig::Setting &smflags = smaterials["flags" + to_string(i)];
+		const libconfig::Setting &smflags = smaterials[("flags" + to_string(i)).c_str()];
 		materials[i].flags = smflags;
 	}
 }
diff --git a/src/configCompat.cpp b/src/configCompat.cpp
index 1aeb262..a72c40f 100644
--- a/src/configCompat.cpp
+++ b/src/configCompat.cpp
@@ -160,19 +160,19 @@ void Common::loadConstantsFromCFGVer0(string cfgFilePath)
 	const Setting &vconstants = constants["Values"];
 	for(int i = 0; i < MaxC; ++i)
 	{
-		C[i] = (int)vconstants[valueConstantsNamesCFGVer0[i]];
+		C[i] = (int)vconstants[valueConstantsNamesCFGVer0[i].c_str()];
 	}
 
 	const Setting &sconstants = constants["Strings"];
 	for(int i = 0; i < MaxS; ++i)
 	{
-		S[i]= (char const*)sconstants[stringConstantsNamesCFGVer0[i]];
+		S[i]= (char const*)sconstants[stringConstantsNamesCFGVer0[i].c_str()];
 	}
 
 	const Setting &hconstants = constants["Hacks"];
 	for(int i = 0; i < MaxH; ++i)
 	{
-		H[i] = (bool)hconstants[hackConstantsNamesVer0[i]];
+		H[i] = (bool)hconstants[hackConstantsNamesVer0[i].c_str()];
 	}
 }
 
diff --git a/src/configHelper.cpp b/src/configHelper.cpp
index fcd1f3f..a63bddc 100644
--- a/src/configHelper.cpp
+++ b/src/configHelper.cpp
@@ -54,15 +54,11 @@ template Uint8 ConfigHelper::getValue<Uint8, const Setting, int>(const Setting &
 
 template Uint8 ConfigHelper::getValue<Uint8, const Setting, char const*>(const Setting &node, char const* index);
 
-template Uint8 ConfigHelper::getValue<Uint8, const Setting, string>(const Setting &node, string index);
-
 // Non-const
 template Uint8 ConfigHelper::getValue<Uint8, Setting, int>(Setting &node, int index);
 
 template Uint8 ConfigHelper::getValue<Uint8, Setting, char const*>(Setting &node, char const* index);
 
-template Uint8 ConfigHelper::getValue<Uint8, Setting, string>(Setting &node, string index);
-
 
 // Since we still need specialisation per value type (Setting::Type),
 // no need to templateify these
@@ -72,7 +68,7 @@ void ConfigHelper::put(Setting &node, string variable, string value)
 	{
 		node.add(variable, Setting::TypeString) = value;
 	} else {
-		Setting &var = node[variable];
+		Setting &var = node[variable.c_str()];
 		var = value;
 	}
 }
@@ -83,7 +79,7 @@ void ConfigHelper::put(Setting &node, string variable, int value)
 	{
 		node.add(variable, Setting::TypeInt) = value;
 	} else {
-		Setting &var = node[variable];
+		Setting &var = node[variable.c_str()];
 		var = value;
 	}
 }
@@ -94,7 +90,7 @@ void ConfigHelper::put(Setting &node, string variable, Uint8 value)
 	{
 		node.add(variable, Setting::TypeInt) = value;
 	} else {
-		Setting &var = node[variable];
+		Setting &var = node[variable.c_str()];
 		var = value;
 	}
 }
@@ -105,7 +101,7 @@ void ConfigHelper::put(Setting &node, string variable, bool value)
 	{
 		node.add(variable, Setting::TypeBoolean) = value;
 	} else {
-		Setting &var = node[variable];
+		Setting &var = node[variable.c_str()];
 		var = value;
 	}
 }
@@ -135,6 +131,6 @@ Setting& ConfigHelper::getSubgroup(Setting &node, string groupName)
 	{
 		node.add(groupName, Setting::TypeGroup);
 	}
-	return node[groupName];
+	return node[groupName.c_str()];
 }
 
diff --git a/src/constants.cpp b/src/constants.cpp
index 7fced6a..cf7bbfc 100644
--- a/src/constants.cpp
+++ b/src/constants.cpp
@@ -523,19 +523,19 @@ void Common::loadConstantsFromCFG(std::string cfgFilePath)
 	const libconfig::Setting &vconstants = constants["Values"];
 	for(int i = 0; i < MaxC; ++i)
 	{
-		C[i] = (int)vconstants[valueConstantsNames[i]];
+		C[i] = (int)vconstants[valueConstantsNames[i].c_str()];
 	}
 
 	const libconfig::Setting &sconstants = constants["Strings"];
 	for(int i = 0; i < MaxS; ++i)
 	{
-		S[i]= (char const*)sconstants[stringConstantsNames[i]];
+		S[i]= (char const*)sconstants[stringConstantsNames[i].c_str()];
 	}
 
 	const libconfig::Setting &hconstants = constants["Hacks"];
 	for(int i = 0; i < MaxH; ++i)
 	{
-		H[i] = (bool)hconstants[hackConstantsNames[i]];
+		H[i] = (bool)hconstants[hackConstantsNames[i].c_str()];
 	}
 }
 
diff --git a/src/gfx/palette.cpp b/src/gfx/palette.cpp
index 3fd08c4..3d3bf22 100644
--- a/src/gfx/palette.cpp
+++ b/src/gfx/palette.cpp
@@ -124,9 +124,9 @@ void Palette::readFromCFG(std::string cfgFilePath)
 
 	for(int i = 0; i < 256; ++i)
 	{
-		entries[i].r = cfgHelp.getValue<Uint8>(spentries, "entries" + to_string(i) + "r");
-		entries[i].g = cfgHelp.getValue<Uint8>(spentries, "entries" + to_string(i) + "g");
-		entries[i].b = cfgHelp.getValue<Uint8>(spentries, "entries" + to_string(i) + "b");
+		entries[i].r = cfgHelp.getValue<Uint8>(spentries, ("entries" + to_string(i) + "r").c_str());
+		entries[i].g = cfgHelp.getValue<Uint8>(spentries, ("entries" + to_string(i) + "g").c_str());
+		entries[i].b = cfgHelp.getValue<Uint8>(spentries, ("entries" + to_string(i) + "b").c_str());
 	}
 }