release: add singleinclude and meson.build to include.zip

This serves as a minimal release-only way to embed json into a project.
Add meson support to this directly, to make it usable standalone as a
meson subproject.

Implements #1672

Makefile

@@ -594,7 +594,7 @@ ChangeLog.md:
 release:
 	rm -fr release_files
 	mkdir release_files
-	zip -9 --recurse-paths -X include.zip $(SRCS)
+	zip -9 --recurse-paths -X include.zip $(SRCS) $(AMALGAMATED_FILE) meson.build
 	gpg --armor --detach-sig include.zip
 	mv include.zip include.zip.asc release_files
 	gpg --armor --detach-sig $(AMALGAMATED_FILE)