Make packaging with gnulib easier

Hi,

since 3.7.10 guile-gnutls seems to require the source tree of gnulib present while building. This makes packaging a bit harder.

I see at least two ways to make it more simple again:

• provide "bootstrapped" tarballs (those who are attached to the release) on a nice link, e.g. somewhere here https://www.gnupg.org/ftp/gcrypt/gnutls/
• allow the usage of the gnulib shipped by the distro. Don't know if thats possible. But the --gnulib-srcdir needs to be adapted, as those files are distributed over the file system (/usr/bin, /usr/share etc...)

~Jonathan