Add _GNU_SOURCE to libevent build for Android builds (#7497)
Add _GNU_SOURCE to libevent build for Android builds (#7497)

The Android NDK doesn't assume `_GNU_SOURCE`, clang only assumes it for
C++ builds, not C builds. libevent uses `pipe2` which in the NDK is only
exposed in the header when these variables are set. In bazel 0.27.0
these implicit use became an error.

More details:

- https://github.com/lyft/envoy-mobile/issues/116
- https://github.com/libevent/libevent/pull/850

Signed-off-by: Keith Smiley <cadcacca0656fa2178c91a0e61e28374cd3a79c3@gmail.com>
eklitzke/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,lyft/envoy,lizan/envoy,eklitzke/envoy,envoyproxy/envoy-wasm,lyft/envoy,envoyproxy/envoy-wasm,istio/envoy,envoyproxy/envoy-wasm,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,eklitzke/envoy,istio/envoy,envoyproxy/envoy-wasm,jrajahalme/envoy,jrajahalme/envoy,istio/envoy,envoyproxy/envoy,jrajahalme/envoy,eklitzke/envoy,jrajahalme/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,istio/envoy,envoyproxy/envoy,istio/envoy,eklitzke/envoy,jrajahalme/envoy,istio/envoy,envoyproxy/envoy,jrajahalme/envoy,envoyproxy/envoy,lizan/envoy,lizan/envoy,lyft/envoy,lizan/envoy,lizan/envoy,lizan/envoy,istio/envoy,eklitzke/envoy