gnu: mingw-w64: Add -winpthreads variants.

This recursive package definition really demonstrates how magical Guix
can be :-)

Try invoking:
    ./pre-inst-env guix build mingw-w64-{x86_64,i686}{,-winpthreads}

* gnu/packages/mingw.scm (make-mingw-w64): Add XGCC, XBINUTILS optional
arguments to specify using a non-default cross-compiler/binutils.  Add
WITH-WINPTHREADS? optional argument to allow building with winpthreads
support.  Adjust accordingly for the new arguments.
(mingw-w64-i686-winpthreads, mingw-w64-x86_64-winpthreads): Add
variables.
* gnu/packages/cross-base.scm (native-libc): Add XGCC, XBINUTILS
key arugments and pass to MAKE-MINGW-W64.
(cross-libc): Pass XGCC and XBINUTILS to NATIVE-LIBC.

gnu/packages/cross-base.scm

@@ -454,7 +454,9 @@ target that libc."
   "Return LIBC cross-built for TARGET, a GNU triplet. Use XGCC and XBINUTILS
 and the cross tool chain."
   (if (cross-newlib? target libc)
-      (native-libc target libc)
+      (native-libc target libc
+                   #:xgcc xgcc
+                   #:xbinutils xbinutils)
       (let ((libc libc))
         (package (inherit libc)
           (name (string-append "glibc-cross-" target))
@@ -511,10 +513,15 @@ and the cross tool chain."
 
 (define* (native-libc target
                      #:optional
-                     (libc glibc))
+                     (libc glibc)
+                     #:key
+                     xgcc
+                     xbinutils)
   (if (target-mingw? target)
       (let ((machine (substring target 0 (string-index target #\-))))
-        (make-mingw-w64 machine))
+        (make-mingw-w64 machine
+                        #:xgcc xgcc
+                        #:xbinutils xbinutils))
       libc))
 
 (define* (cross-newlib? target