fix declarations that were missing due to conditional includes

Paul Vines was running into some missing declarations (strdup and struct timespec) when running in an Ubuntu 18.04 container on Kudu's server. I was able to repro this on a stock (bare-metal) 18.04 install. The issue is that both of these are conditionally defined, and -std=c99 hides them unless explicitly told otherwise. This patch tells the compiler otherwise.