Merge pull request #188 from Unity-Technologies/fix_build_issue

delete SDKROOT in the env if any

engine/Scripts/lib_build.py

     global bitcode
     dest_folder=""
 
+    #delete SDKRoot env if any. It may conflict with the real sysroot path generated by Bee
+    if 'SDKROOT' in os.environ:
+        del os.environ['SDKROOT']
+
     print("\nStarting build engine...")
     if platform == "windows":
         dest_folder = "x86_64"