gnu: r: Apply patch to fix linking against R.

* gnu/packages/patches/r-fix-15899.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/statistics.scm (r)[source]: Use it.

gnu-system.am

@@ -542,6 +542,7 @@ dist_patch_DATA =						\
   gnu/packages/patches/qt4-tests.patch				\
   gnu/packages/patches/qt5-conflicting-typedefs.patch		\
   gnu/packages/patches/qt5-runpath.patch			\
+  gnu/packages/patches/r-fix-15899.patch			\
   gnu/packages/patches/ratpoison-shell.patch			\
   gnu/packages/patches/readline-link-ncurses.patch		\
   gnu/packages/patches/ripperx-libm.patch			\

gnu/packages/patches/r-fix-15899.patch

@@ -0,0 +1,17 @@
+Without the "extern" keyword external applications linking against R (such as
+Shogun, for example) might not be linkable.
+
+See https://bugs.r-project.org/bugzilla3/show_bug.cgi?id=15899 for details
+about this bug.
+
+--- a/src/include/Rinterface.h	(revision 66251)
++++ b/src/include/Rinterface.h	(working copy)
+@@ -84,7 +84,7 @@
+ void fpu_setup(Rboolean);
+ 
+ /* in unix/system.c */
+-int R_running_as_main_program; 
++extern int R_running_as_main_program; 
+ 
+ #ifdef CSTACK_DEFNS
+ /* duplicating Defn.h */

gnu/packages/statistics.scm

@@ -50,7 +50,8 @@
                                   version ".tar.gz"))
               (sha256
                (base32
-                "0dagyqgvi8i3nw158qi2zpwm04s4ffzvnmk5niaksvxs30zrbbpm"))))
+                "0dagyqgvi8i3nw158qi2zpwm04s4ffzvnmk5niaksvxs30zrbbpm"))
+              (patches (list (search-patch "r-fix-15899.patch")))))
     (build-system gnu-build-system)
     (arguments
      `(#:make-flags