build: added build-thankful target

Author: ErikBjare

Makefile

@@ -7,3 +7,12 @@ serve:
 build:
 	bundle exec jekyll build
 	touch _site/.nojekyll
+
+# We need to build the thankful page separately,
+# and push it manually to `thankful:gh-pages`,
+# because it's in a subdirectory named the same as the repo,
+# which GitHub Pages doesn't like.
+build-thankful:
+	bundle exec jekyll build -d _site/thankful
+	mv _site/thankful/thankful/* _site/thankful
+	touch _site/thankful/.nojekyll