restore behaviour where DEVELOPER_DIR pointing not Xcode.app still works (#544)

xcode-select is able to handle DEVELOPER_DIR pointing to Xcode.app even though
historically it should be pointing to the Contents/Developer sub directory.
Restore previous behaviour where setting DEVELOPER_DIR to root xcode folder still works in ios-deploy.
diff --git a/src/ios-deploy/ios-deploy.m b/src/ios-deploy/ios-deploy.m
index 40a2cea..06eb792 100644
--- a/src/ios-deploy/ios-deploy.m
+++ b/src/ios-deploy/ios-deploy.m
@@ -359,6 +359,15 @@
         
         if (env_dev_path && strlen(env_dev_path) > 0) {
             strcpy(xcode_dev_path, env_dev_path);
+            // DEVELOPER_DIR should refer to Xcode.app/Contents/Developer, but
+            // xcode-select and friends have an extension to fix the path, if it points to Xcode.app/.
+            static char dev_subdir[256] = { '\0' };
+            strcat(strcat(dev_subdir, env_dev_path), "/Contents/Developer");
+            struct stat sb;
+            if (stat(dev_subdir, &sb) == 0)
+            {
+                strcpy(xcode_dev_path, dev_subdir);
+            }
         } else {
             FILE *fpipe = NULL;
             char *command = "xcode-select -print-path";