gnu: racket: Update to 6.12.

* gnu/packages/patches/racket-minus_zero_p-disable-xform.patch: New file.
* gnu/packages/scheme.scm (racket): Update version and patch.

gnu/packages/patches/racket-minus_zero_p-disable-xform.patch

@@ -0,0 +1,10 @@
+--- a/src/racket/src/number.c	2018-02-24 18:16:32.031698079 -0600
++++ b/src/racket/src/number.c	2018-02-24 18:15:41.055445301 -0600
+@@ -1775,6 +1775,7 @@
+ }
+ 
+ XFORM_NONGCING static MZ_INLINE int minus_zero_p(double d)
++  XFORM_SKIP_PROC
+ {
+ #ifdef MZ_IS_NEG_ZERO
+   return MZ_IS_NEG_ZERO(d);

gnu/packages/scheme.scm

@@ -405,7 +405,7 @@ implementation techniques and as an expository tool.")
 (define-public racket
   (package
     (name "racket")
-    (version "6.11")
+    (version "6.12")
     (source (origin
              (method url-fetch)
              (uri (list (string-append "http://mirror.racket-lang.org/installers/"
@@ -415,7 +415,10 @@ implementation techniques and as an expository tool.")
                          version "/racket-" version "-src.tgz")))
              (sha256
               (base32
-               "1nk7705x24jjlbqqhj8yvbgqkfscxx3m81bry1g56kjxysjmf3sw"))))
+               "0cwcypzjfl9py1s695mhqkiapff7c1w29llsmdj7qgn58wl0apk5"))
+             (patches (search-patches
+                       ;; See: https://github.com/racket/racket/issues/1962
+                       "racket-minus_zero_p-disable-xform.patch"))))
     (build-system gnu-build-system)
     (arguments
      '(#:phases