gnu: Add libcxx.

* gnu/packages/llvm.scm (libcxx): New variable.

gnu/packages/llvm.scm

@@ -3,7 +3,7 @@
 ;;; Copyright © 2015 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2015, 2017, 2018, 2019 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2016 Dennis Mungai <dmngaie@gmail.com>
-;;; Copyright © 2016, 2018 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2016, 2018, 2019 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2017 Roel Janssen <roel@gnu.org>
 ;;; Copyright © 2018, 2019 Marius Bakke <mbakke@fastmail.com>
 ;;; Copyright © 2018 Tobias Geerinckx-Rice <me@tobias.gr>
@@ -259,6 +259,29 @@ project includes the Clang front end, the Clang static analyzer, and several
 code analysis tools.")
     (license license:ncsa)))
 
+(define-public libcxx
+  (package
+    (name "libcxx")
+    (version (package-version llvm))
+    (source
+     (origin
+       (method url-fetch)
+       (uri (string-append "http://llvm.org/releases/"
+                           version "/libcxx-" version ".src.tar.xz"))
+       (sha256
+        (base32
+         "0rzw4qvxp6qx4l4h9amrq02gp7hbg8lw4m0sy3k60f50234gnm3n"))))
+    (build-system cmake-build-system)
+    (native-inputs
+     `(("clang" ,clang)
+       ("llvm" ,llvm)))
+    (home-page "https://libcxx.llvm.org")
+    (synopsis "C++ standard library")
+    (description
+     "This package provides an implementation of the C++ standard library for
+use with Clang, targeting C++11, C++14 and above.")
+    (license license:expat)))
+
 (define-public clang-runtime
   (clang-runtime-from-llvm
    llvm