Separate build script from install script #4458

The --build-from-source flag is now deprecated and will soon
be removed along with the need to define an install script.

This will remove a whole category of package manager
warnings about install scripts and "built" dependencies.

Most people don't need to build sharp from source, but for
those that do, a suitable method is now something like:

$ npm install package-that-depends-on-sharp
$ npm explore sharp -- npm run build

install/build.js

@@ -0,0 +1,36 @@
+// Copyright 2013 Lovell Fuller and others.
+// SPDX-License-Identifier: Apache-2.0
+
+const {
+  useGlobalLibvips,
+  globalLibvipsVersion,
+  log,
+  spawnRebuild,
+} = require('../lib/libvips');
+
+log('Attempting to build from source via node-gyp');
+log('See https://sharp.pixelplumbing.com/install#building-from-source');
+
+try {
+  const addonApi = require('node-addon-api');
+  log(`Found node-addon-api ${addonApi.version || ''}`);
+} catch (_err) {
+  log('Please add node-addon-api to your dependencies');
+  process.exit(1);
+}
+try {
+  const gyp = require('node-gyp');
+  log(`Found node-gyp ${gyp().version}`);
+} catch (_err) {
+  log('Please add node-gyp to your dependencies');
+  process.exit(1);
+}
+
+if (useGlobalLibvips(log)) {
+  log(`Detected globally-installed libvips v${globalLibvipsVersion()}`);
+}
+
+const status = spawnRebuild();
+if (status !== 0) {
+  process.exit(status);
+}