gnu: mingw-w64: Fix home page and avoid further confusion.

MinGW-w64 was forked (in rather dramatic fashion) from the original
MinGW.  The two projects remain distinct and MinGW-w64 seems by far the
actively developed of the two.

* gnu/packages/mingw.scm (mingw-w64)[home-page]: Point to the correct
upstream.  Use HTTPS.
[description]: Expand and briefly note the relationship between the
two projects.  Use @dfn.

gnu/packages/mingw.scm

@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2016 Jan Nieuwenhuizen <janneke@gnu.org>
+;;; Copyright © 2018 Tobias Geerinckx-Rice <me@tobias.gr>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -76,9 +77,16 @@
        #:make-flags (list "DEFS=-DHAVE_CONFIG_H -D__MINGW_HAS_DXSDK=1")
        #:tests? #f ; compiles and includes glibc headers
        #:strip-binaries? #f))
-    (home-page "http://mingw.org")
+    (home-page "https://mingw-w64.org")
     (synopsis "Minimalist GNU for Windows")
-    (description "MinGW provides a complete programming tool set which is
-suitable for the development of native MS-Windows applications, and which does
-not depend on any 3rd-party C-Runtime dlls.")
+    (description
+     "Minimalist GNU for Windows (@dfn{MinGW}) is a complete software
+development environment for creating native Microsoft Windows applications.
+
+It includes a set of Windows-specific header files and static import libraries
+which enable the use of the Windows API.  It does not rely on any third-party C
+runtime dynamic-link libraries (@dfn{DLL}s).
+
+Mingw-w64 is an advancement of the original mingw.org project and provides
+several new APIs such as DirectX and DDK, and 64-bit support.")
     (license license:fdl1.3+)))