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 </td></tr></table>
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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());
 	}
 }