Add infrastructure to build and publish as wasm32 (#3840)

Co-authored-by: Ingvar Stepanyan <me@rreverser.com>

src/emscripten/common.gypi

@@ -0,0 +1,40 @@
+# Copyright 2013 Lovell Fuller and others.
+# SPDX-License-Identifier: Apache-2.0
+
+{
+  'variables': {
+    'OS': 'emscripten'
+  },
+  'target_defaults': {
+    'default_configuration': 'Release',
+    'type': 'executable',
+    'cflags': [
+      '-pthread',
+      '-sDEFAULT_TO_CXX=0'
+    ],
+    'cflags_cc': [
+      '-pthread'
+    ],
+    'ldflags': [
+      '--js-library=<!(node -p "require(\'emnapi\').js_library")',
+      '-sAUTO_JS_LIBRARIES=0',
+      '-sAUTO_NATIVE_LIBRARIES=0',
+      '-sNODEJS_CATCH_EXIT=0',
+      '-sNODEJS_CATCH_REJECTION=0'
+    ],
+    'defines': [
+      '__STDC_FORMAT_MACROS',
+      'BUILDING_NODE_EXTENSION',
+      'EMNAPI_WORKER_POOL_SIZE=1'
+    ],
+    'include_dirs': [
+      '<!(node -p "require(\'emnapi\').include")'
+    ],
+    'sources': [
+      '<!@(node -p "require(\'emnapi\').sources.map(x => JSON.stringify(path.relative(process.cwd(), x))).join(\' \')")'
+    ],
+    'configurations': {
+      'Release': {}
+    }
+  }
+}

src/emscripten/pre.js

@@ -0,0 +1,19 @@
+// Copyright 2013 Lovell Fuller and others.
+// SPDX-License-Identifier: Apache-2.0
+
+/* global Module, ENV, _vips_shutdown, _uv_library_shutdown */
+
+Module.preRun = () => {
+  ENV.VIPS_CONCURRENCY = Number(process.env.VIPS_CONCURRENCY) || 1;
+};
+
+Module.onRuntimeInitialized = () => {
+  module.exports = Module.emnapiInit({
+    context: require('@emnapi/runtime').getDefaultContext()
+  });
+
+  process.once('exit', () => {
+    _vips_shutdown();
+    _uv_library_shutdown();
+  });
+};