Fail the build early on unsupported stacks

In order to prevent the build completing apparently successfully, but
the app fail to boot at runtime due to stack incompatibility.

At first glance this would seem unnecessary due to the stack-specific
URL meaning the `curl` would 404 on supported stacks. However heroku#165
means the Cedar-14 binary is installed on all stacks, and on Heroku-20
causes the failures at runtime seen in heroku#166.

Future PRs will fix the curl/binary handling to use stack-specific URLs,
however it's still nicer to explicitly handle unsupported stacks with a
clear error message than a 404.

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+* [#177](https://github.com/heroku/heroku-buildpack-static/pull/177) Fail the build early on unsupported stacks
 
 ## v4 (2019-09-18)
 

bin/compile

@@ -8,6 +8,16 @@ cache_dir=$2
 env_dir=$3
 bp_dir=$(dirname $(dirname $0))
 
+case "${STACK}" in
+  cedar-14|heroku-16|heroku-18)
+    # Supported stack
+    ;;
+  *)
+    echo "Stack ${STACK} is not supported!"
+    exit 1
+    ;;
+esac
+
 fetch_nginx_tarball() {
     local version="1.9.7"
     local tarball_file="nginx-$version.tgz"