Replace SDL_strdup() by strdup()

The functions SDL_malloc(), SDL_free() and SDL_strdup() were used only
because strdup() was not available everywhere.

Now that it is available, use the native version of these functions.

app/src/sys/unix/process.c

@@ -156,7 +156,7 @@ get_executable_path(void) {
         return NULL;
     }
     buf[len] = '\0';
-    return SDL_strdup(buf);
+    return strdup(buf);
 #else
     // in practice, we only need this feature for portable builds, only used on
     // Windows, so we don't care implementing it for every platform

app/src/sys/win/process.c

@@ -41,7 +41,7 @@ process_execute(const char *const argv[], HANDLE *handle) {
 
     if (!CreateProcessW(NULL, wide, NULL, NULL, FALSE, 0, NULL, NULL, &si,
                         &pi)) {
-        SDL_free(wide);
+        free(wide);
         *handle = NULL;
         if (GetLastError() == ERROR_FILE_NOT_FOUND) {
             return PROCESS_ERROR_MISSING_BINARY;
@@ -49,7 +49,7 @@ process_execute(const char *const argv[], HANDLE *handle) {
         return PROCESS_ERROR_GENERIC;
     }
 
-    SDL_free(wide);
+    free(wide);
     *handle = pi.hProcess;
     return PROCESS_SUCCESS;
 }
@@ -105,7 +105,7 @@ is_regular_file(const char *path) {
 
     struct _stat path_stat;
     int r = _wstat(wide_path, &path_stat);
-    SDL_free(wide_path);
+    free(wide_path);
 
     if (r) {
         perror("stat");