gnu: gnutls: Add "guile3.0-gnutls" variant.

* gnu/packages/tls.scm (gnutls-3.6.10, guile3.0-gnutls): New variables.

gnu/packages/tls.scm

@@ -263,6 +263,39 @@ required structures.")
     (inputs `(("unbound" ,unbound)
               ,@(package-inputs gnutls)))))
 
+(define gnutls-3.6.10
+  ;; This is for 'guile3.0-gnutls', below.  Version 3.6.10 is the first to
+  ;; introduce Guile 2.9/3.0 support.
+  (package
+    (inherit gnutls)
+    (version "3.6.10")
+    (source (origin
+              (inherit (package-source gnutls))
+              (uri (string-append "mirror://gnupg/gnutls/v"
+                                  (version-major+minor version)
+                                  "/gnutls-" version ".tar.xz"))
+              (sha256
+               (base32
+                "14r2h73yfj66cm14k9mnb3kgzq5a7qjg5b31m53bf19vcxkwmwxi"))))))
+
+(define-public guile3.0-gnutls
+  (package
+    (inherit gnutls-3.6.10)
+    (name "guile3.0-gnutls")
+    (arguments
+     (substitute-keyword-arguments (package-arguments gnutls-3.6.10)
+       ((#:phases phases '%standard-phases)
+        `(modify-phases ,phases
+           (add-before 'build 'leave-guile-stdout-open
+             (lambda _
+               ;; Work around <https://bugs.gnu.org/38348>.
+               (substitute* "guile/Makefile"
+                 (("out=-") "out=/dev/null"))
+               #t))))))
+    (inputs `(("guile" ,guile-next)
+              ,@(alist-delete "guile"
+                              (package-inputs gnutls-3.6.10))))))
+
 (define-public openssl
   (package
    (name "openssl")