gnu: gRPC: Re-add custom c-ares variant.

Commit e8393ebe46 updated c-ares without knowing
that cmake-build-system was the most important feature of 'c-ares-next'.

* gnu/packages/adns.scm (c-ares/cmake): New hidden package.
* gnu/packages/machine-learning.scm (grpc)[inputs]: Change C-ARES to C-ARES/CMAKE.

gnu/packages/adns.scm

@@ -2,6 +2,7 @@
 ;;; Copyright © 2014 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2015, 2016, 2018 Efraim Flashner <efraim@flashner.co.il>
 ;;; Copyright © 2018 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2019 Marius Bakke <mbakke@fastmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -82,3 +83,17 @@ (define-public c-ares
 The primary examples of such applications are servers which communicate with
 multiple clients and programs with graphical user interfaces.")
     (license (x11-style "https://c-ares.haxx.se/license.html"))))
+
+;; gRPC requires a c-ares built with CMake in order to get the .cmake modules.
+;; We can not build c-ares itself with CMake because that would introduce a
+;; circular dependency through nghttp2.
+;; XXX: It would be nice if we could extract the modules somehow and make them
+;; work with the "normal" c-ares package instead of building a whole new library.
+(define-public c-ares/cmake
+  (hidden-package
+   (package
+     (inherit c-ares)
+     (build-system cmake-build-system)
+     (arguments
+      `(;; XXX: Tests require name resolution (the normal variant runs no tests).
+        #:tests? #f)))))

gnu/packages/machine-learning.scm

@@ -1292,7 +1292,7 @@ (define-public grpc
              "-DgRPC_SSL_PROVIDER=package"
              "-DgRPC_PROTOBUF_PROVIDER=package")))
     (inputs
-     `(("c-ares" ,c-ares)
+     `(("c-ares" ,c-ares/cmake)
        ("openssl" ,openssl)
        ("zlib" ,zlib)))
     (native-inputs