gnu11 is the default now. am: f0096d9cd2

Original change: https://android-review.googlesource.com/c/platform/external/pthreadpool/+/2097114

Change-Id: I32b08ce5f7e3e5c771cd28e01a1574e74c9ae58b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>