Summary:
Improve it in two ways:
1. tools/check_format_compatible.sh is not friendly to run outside FB environment. remove the hard-coded http proxy setting. Instead, move it to Legocastle configuration
2. Always disable warning as error, so that older build is more likely to pass.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6702

Test Plan: Run the test and make sure at least it doesn't break.

Reviewed By: riversand963

Differential Revision: D21033329

fbshipit-source-id: 88b4ec1ec49547b772790050a165466bdc4a62a0