gnutls is also just wrapping gcrypt, but we don't need any of the TLS related functionality. We just need to be able to decrypt AES128-CBC.
All aclocal .m4 files we need are put here and cat'd to acinclude.m4 in the source root. Official ones (taken from the relevant devel packages) are named as-is, unofficial ones (or changed ones) get a gst-prefix.