add missing includes and defines for linux compilation

fix a warning with string cast

src/Menu.cpp

@@ -7,6 +7,7 @@
 
 #include <assert.h>
 #include <dirent.h>
+#include <cctype>
 
 #ifdef __APPLE__
 #include <OpenGL/gl.h>

src/SkeletalModel.cpp

@@ -6,6 +6,7 @@
  * \author xythobuz
  */
 
+#include <cstddef>
 #include "SkeletalModel.h"
 
 SkeletalModel::SkeletalModel() {

src/World.cpp

@@ -5,6 +5,7 @@
  * \author Mongoose
  */
 
+#include <cstdio>
 #include <math.h>
 #include <assert.h>
 

src/utils/strings.cpp

@@ -11,7 +11,7 @@
 #include <string.h>
 #include <assert.h>
 
-#if defined(unix) || defined(__APPLE__)
+#if defined(unix) || defined(__APPLE__) || defined(__linux__)
 #include <wordexp.h>
 #endif
 
@@ -31,7 +31,7 @@ char *stringRemoveQuotes(const char *s) {
 }
 
 char *stringReplace(const char *s, const char *search, const char *replace) {
-    char *tmp = strstr(s, search);
+    char *tmp = strstr((char *)s, search);
     if (tmp == NULL)
         return NULL;
     size_t offset = tmp - s;
@@ -130,7 +130,7 @@ char *fullPath(const char *path, char end) {
     assert(path[0] != '\0');
 
     if (path[0] == '~') {
-#if defined(unix) || defined(__APPLE__)
+#if defined(unix) || defined(__APPLE__) || defined(__linux__)
 
 #ifdef __APPLE__
         // Workaround for Mac OS X. See: