[build] Add build system for submodules

Vendorize the submodules and a simple CMake build
system for all them. The dependencies will inherit
compilation options for core.

The goal is to make Mapbox GL Core completely self
contained with a simple offline build.

scripts/nitpick/vendoring.js

@@ -1,38 +0,0 @@
-#!/usr/bin/env node
-const nitpick = require('.');
-const fs = require('fs');
-const {execSync} = require('child_process');
-
-const head = process.env['CIRCLE_SHA1'];
-const mergeBase = process.env['CIRCLE_MERGE_BASE'];
-if (!mergeBase) {
-    console.log('No merge base available.');
-    return;
-}
-
-const checkAll = process.argv.indexOf('--check-all') >= 2;
-
-// Run the vendoring script for all vendored packages that were modified in this PR, and check for changes.
-fs.readdirSync('vendor')
-  .filter(name => name[0] != '.')
-  .filter(name => fs.statSync(`vendor/${name}`).isDirectory())
-  .filter(name => checkAll || execSync(`git diff --shortstat ${mergeBase} ${head} -- vendor/${name}`).toString().trim())
-  .forEach(name => {
-    execSync(`scripts/vendor/${name}.sh`);
-
-    // List missing files
-    var missing = execSync(`git ls-files --others --exclude-standard -- vendor/${name}`).toString().trim();
-    if (!missing.length) {
-      nitpick.ok(`All files vendored for ${name} are checked in`);
-    } else {
-      nitpick.fail(`These vendored files for ${name} are not checked in:`, missing);
-    }
-
-    // Diff existing files
-    const diff = execSync(`git -c color.ui=always diff -- vendor/${name}`).toString().trim();
-    if (!diff.length) {
-      nitpick.ok(`All files vendored for ${name} are unmodified`);
-    } else {
-      nitpick.fail(`These vendored files for ${name} have modifications:`, diff);
-    }
-  });