guix: Reinstate make-ssp-fixed-gcc

Unfortunately, gcc is still not smart enough to detect whether or not
mingw-w64 provides ssp, so let's put it back just for mingw-w64.

contrib/guix/manifest.scm

@@ -24,6 +24,17 @@
              (guix profiles)
              (guix utils))
 
+(define (make-ssp-fixed-gcc xgcc)
+  "Given a XGCC package, return a modified package that uses the SSP function
+from glibc instead of from libssp.so. Taken from:
+http://www.linuxfromscratch.org/hlfs/view/development/chapter05/gcc-pass1.html"
+  (package
+   (inherit xgcc)
+   (arguments
+    (substitute-keyword-arguments (package-arguments xgcc)
+      ((#:make-flags flags)
+       `(cons "gcc_cv_libc_provides_ssp=yes" ,flags))))))
+
 (define (make-gcc-rpath-link xgcc)
   "Given a XGCC package, return a modified package that replace each instance of
 -rpath in the default system spec that's inserted by Guix with -rpath-link"