Summary:
In fbcode when we build with clang7++, although -faligned-new is available in compile phase, we link with an older version of libstdc++.a and it doesn't come with aligned-new support (e.g. `nm libstdc++.a | grep align_val_t` return empty). In this case the previous -faligned-new detection can pass but will end up with link error. Fixing it by only have the detection for non-fbcode build.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4576
Differential Revision: D10500008
Pulled By: yiwu-arbug
fbshipit-source-id: b375de4fbb61d2a08e54ab709441aa8e7b4b08cf
742302a1a3