package: make aws-sdk-cpp build input optional

I have added an option to turn off this build input because I'm much
more comfortable when I don't have that type of thing on my computer.
Its default value is true in order to avoid impacting anyone who depends
on AWS features.

Change-Id: Ic57f3c9b9468f422e9fbdcf3ba0fe96177631067

package.nix

@@ -80,7 +80,11 @@ let
   versionJson = builtins.fromJSON (builtins.readFile ./version.json);
   version = versionJson.version + versionSuffix;
 
-  aws-sdk-cpp-nix = aws-sdk-cpp.override {
+  aws-sdk-cpp-nix =
+    if aws-sdk-cpp == null then
+      null
+    else
+      aws-sdk-cpp.override {
         apis = [
           "s3"
           "transfer"