Fix Darwin builds for non-standard environments (e.g. Nix)

Author: martinjlowm

aws-lc-sys/builder/cc_builder.rs

@@ -383,6 +383,8 @@ impl CcBuilder {
             let mut new_cflags = original_cflags.clone();
             // The `_FORTIFY_SOURCE` macro often requires optimizations to also be enabled, so unset it.
             new_cflags.push_str(" -O0 -Wp,-U_FORTIFY_SOURCE");
+            // Certain MacOS system headers are guarded by _POSIX_C_SOURCE and _DARWIN_C_SOURCE
+            new_cflags.push_str(" -D_DARWIN_C_SOURCE");
             set_env_for_target("CFLAGS", &new_cflags);
             // cc-rs currently prioritizes flags provided by CFLAGS over the flags provided by the build script.
             // The environment variables used by the compiler are set when `get_compiler` is called.