Modernise C++ to avoid errors in C++14 mode.
See also: https://bugs.gentoo.org/show_bug.cgi?id=600084

--- a/src/main.cpp
+++ b/src/main.cpp
@@ -66,7 +66,7 @@
 
 void writeconfig(const map& game_map) {
 	ofstream config((ostring)TuxHomeDirectory + "/config");	// open config file
-	if(config == NULL) {	// error check
+	if(!config) {	// error check
 		cout << "Warning: Couldn't write to file " << (ostring)TuxHomeDirectory + "/config" << endl;
 		return;
 	}
@@ -124,7 +124,7 @@
 
 void readconfig(class map& game_map) {
 	ifstream config((ostring)TuxHomeDirectory + "/config");	// open config file
-	if(config==0) {	// error check
+	if(!config) {	// error check
 		cout << "Warning: Couldn't find configuration file " << (ostring)TuxHomeDirectory + "/config" << ". Using default values." << endl;
 		return;
 	}
--- a/src/map.cpp
+++ b/src/map.cpp
@@ -346,13 +346,13 @@
 	cout << "-- copy map --" << endl;		// print status message
 	ifstream in_file(path);	// open source file
 
-	if(in_file == 0) {	// error checking
+	if(!in_file) {	// error checking
 		cout << "Couldn't open sourcefile \"" << filename << "\"" << endl;
 		cout << endl << "-- error in copymap --" << endl;
 		return 1;
 	}
 
-	if(out_file == 0) {	// error checking
+	if(!out_file) {	// error checking
 		cout << "Couldn't open target file \"" << temp_path << "\" for writing " << endl;
 		cout << endl << "-- error in copymap --" << endl;
 		return 1;
@@ -390,7 +390,7 @@
 		path = mapfolder;
 		path += filename;
 		file.open(path);	// open file
-		if(file == NULL) {
+		if(!file) {
 			cout << "map::savemap :  error while saving map to file '" << path << "'" << endl;
 			return 1;
 		}
@@ -402,7 +402,7 @@
 		path = savefolder;
 		path += filename;
 		file.open(path);	// open file
-		if(file == NULL) {
+		if(!file) {
 			cout << "map::savemap : error while saving game to file '" << path << "'" << endl;
 			return 1;
 		}
--- a/src/menu.cpp
+++ b/src/menu.cpp
@@ -119,7 +119,7 @@
 
 // add a selection box
 
-class element* menu_mgm::add_box(int x, int y, const ostring& text, const ostring& value, bool selectable, int size, int xgroup, int ygroup, int max, int width, int height, unsigned char r, unsigned char g, unsigned char b, int value_type, bool dependency, char* depend) {
+class element* menu_mgm::add_box(int x, int y, const ostring& text, const ostring& value, bool selectable, int size, int xgroup, int ygroup, int max, int width, int height, unsigned char r, unsigned char g, unsigned char b, int value_type, bool dependency, const char* depend) {
 	class element& newone = add();
 	newone.value = value;
 	newone.posx = x;
--- a/src/menu.h
+++ b/src/menu.h
@@ -56,7 +56,7 @@
 		void check_custom_parameters();	// check if parameters are okay
 		ostring keytoa(SDLKey);	// cast SDLKey to ASCII
 		class element* add_text(int, int, const ostring&, bool, int, int =-1, int =-1, unsigned char =0, unsigned char =0, unsigned char = 0, int = -1, int = -1);	// add a text element
-		class element* add_box(int, int, const ostring&, const ostring&, bool, int, int, int, int, int, int, unsigned char, unsigned char, unsigned char, int = 0, bool =false, char* =0);	// add a box element
+		class element* add_box(int, int, const ostring&, const ostring&, bool, int, int, int, int, int, int, unsigned char, unsigned char, unsigned char, int = 0, bool =false, const char* =0);	// add a box element
 		class element* add_select(int, int, const ostring&, const ostring&, bool, int, int, int, const char* oneoftwo=0);	// add a select element
 		void draw_window();	// draw the current menu screen with all elements
 		void selection_mgm(char);	// process user input for menu navigation
--- a/src/surface.cpp
+++ b/src/surface.cpp
@@ -92,7 +92,7 @@
 	else file_tmp = file;
 
 	TTF_Font *font = TTF_OpenFont(file_tmp, size);
-	SDL_Color color = {r, g, b, 0};
+	SDL_Color color = {(Uint8)r, (Uint8)g, (Uint8)b, 0};
 
 	area = TTF_RenderText_Solid(font, text, color);
 
@@ -118,7 +118,7 @@
 	SDL_Surface* text_surface;
 	SDL_Rect temp;
 	TTF_Font *font_tmp = TTF_OpenFont(font, size);
-	SDL_Color farbe = {r, g, b, 0};
+	SDL_Color farbe = {(Uint8)r, (Uint8)g, (Uint8)b, 0};
 	text_surface = TTF_RenderText_Solid(font_tmp, text, farbe);
 	TTF_CloseFont(font_tmp);
 	temp = pos;