Made libyaml into a submodule and now builds from that.

Also attempts to fix the Github-specific CI configuration for this
rgem.

.github/workflows/ubuntu.yml

@@ -2,6 +2,12 @@ name: ubuntu
 on: [push, pull_request]
 jobs:
   gcc:
+    env:
+      # 'set-env' has been disabled by default because it's a
+      # potential security issue.  See https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/.
+      # We re-enable it here for now because the problem does not
+      # appear to affect us.
+      ACTIONS_ALLOW_UNSECURE_COMMANDS: true
     runs-on: ubuntu-latest
     steps:
       - name: Install libraries

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "third_party/libyaml"]
+	path = third_party/libyaml
+	url = https://github.com/yaml/libyaml.git

README.md

@@ -28,6 +28,13 @@ else
 end
 ```
 
+### Getting libyaml
+
+If you have a compiled version of `libyaml` available on your system, you can use it by setting the environment variable `MRUBY_YAML_USE_SYSTEM_LIBRARY` to a non-empty value and ensuring your compiler can find the library.
+
+Otherwise, Rake will attempt to compile it from sources obtained from the offical `libyaml` GitHub repository.  This requires that you have `autoconf` installed.
+
+
 ### Documentation
 
 #### `YAML.load(yaml_str)`

mrbgem.rake

@@ -20,18 +20,22 @@ MRuby::Gem::Specification.new('mruby-yaml') do |spec|
   use_system_library = ENV.fetch('MRUBY_YAML_USE_SYSTEM_LIBRARY', '') != ''
 
   unless use_system_library
-    yaml_version = "0.2.2"
-    yaml_dir = "#{build_dir}/yaml-#{yaml_version}"
+    yaml_base_dir = "libyaml"
+    yaml_dir = File.join(build_dir, yaml_base_dir)
 
     FileUtils.mkdir_p build_dir
 
-    if ! File.exists? yaml_dir
-      Dir.chdir(build_dir) do
-        e = {}
-        tar_zxf = (RUBY_PLATFORM.match(/solaris/) ? 'gzip -d | tar xf -' : 'tar zxf -')
-        run_command e, "curl -L https://pyyaml.org/download/libyaml/yaml-#{yaml_version}.tar.gz | #{tar_zxf}"
-        run_command e, "mkdir #{yaml_dir}/build"
-      end
+    # We build libyaml in the gem's build directory, which means
+    # copying the sources from the repo.
+    if ! File.exists? "#{yaml_dir}"
+
+      # But first, we generate the configure script. This requires GNU
+      # autoconf to be installed.
+      Dir.chdir(File.join(spec.dir, 'third_party', yaml_base_dir)) {
+        run_command({}, "./bootstrap")
+      }
+
+      FileUtils.cp_r File.join(spec.dir, 'third_party', yaml_base_dir), build_dir
     end
 
     if ! File.exists? "#{yaml_dir}/build/lib/libyaml.a"