Pin to C99. am: 3c8f4960c7

Original change: https://android-review.googlesource.com/c/platform/external/libconfig/+/3323953

Change-Id: I577272153bf452c51345f850b531c26007b69950
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>