Prevent building libmemcached on stacks that don't vendor it (#964)

The `libmemcached` package is available in the base stack image for all
stacks newer than `cedar-14`, so at buildpack compile time the vendor
step is skipped for those stacks:
https://github.com/heroku/heroku-buildpack-python/blob/106f2997fa124852a2a35ee8bfa604ad20c47988/bin/steps/pylibmc#L12-L15

As such, it is not necessary to run the libmemcached bob-builder formula
on newer stacks. The conditional has been updated so it correctly handles
heroku-18 and also the upcoming heroku-20.

An exit code of 1 has been used, otherwise `bob upload` will build and
then upload a zero byte archive to S3, which will go unused.

(This is in comparison to bob formulas that are nested, where an exit
code of 0 is actually desirable, since it allows skipping steps.)

Refs W-7485877.

Co-authored-by: Joe Kutner <jpkutner@gmail.com>

builds/libraries/vendor/libmemcache

@@ -3,9 +3,9 @@
 
 OUT_PREFIX=$1
 
-# Skip the build for heroku-16.
-if [[ $S3_PREFIX == "heroku-16" ]]; then
-    exit 0
+if [[ $S3_PREFIX != "cedar-14" ]]; then
+    echo "libmemcached only needs to be built for cedar-14, since newer stacks include it in the base image"
+    exit 1
 fi
 
 # fail hard