gnu: Use 'this-package' as a native input instead of 'self-native-input?'.

* gnu/packages/file.scm (file)[self-native-input?]: Remove.
[native-inputs]: New field.
* gnu/packages/guile.scm (guile-1.8)[self-native-input?]: Remove.
[native-inputs]: New field.
(guile-2.0)[self-native-input?]: Remove.
[native-inputs]: Add THIS-PACKAGE when (%current-target-system) is true.
* gnu/packages/ncurses.scm (ncurses)[self-native-input?]: Remove.
[native-inputs]: Add THIS-PACKAGE when (%current-target-system) is true.
* gnu/packages/python-xyz.scm (python-file)[self-native-input?]: Remove.
[native-inputs]: New field.

gnu/packages/file.scm

@@ -43,7 +43,9 @@
 
    ;; When cross-compiling, this package depends upon a native install of
    ;; itself.
-   (self-native-input? #t)
+   (native-inputs (if (%current-target-system)
+                      `(("self" ,this-package))
+                      '()))
 
    (synopsis "File type guesser")
    (description

gnu/packages/guile.scm

@@ -103,6 +103,12 @@
                       (substitute* "ice-9/popen.scm"
                         (("/bin/sh") (which "sh")))
                       #t)))))
+
+   ;; When cross-compiling, a native version of Guile itself is needed.
+   (native-inputs (if (%current-target-system)
+                      `(("self" ,this-package))
+                      '()))
+
    (inputs `(("gawk" ,gawk)
              ("readline" ,readline)))
 
@@ -111,9 +117,6 @@
    (propagated-inputs `(("gmp" ,gmp)
                         ("libltdl" ,libltdl)))
 
-   ;; When cross-compiling, a native version of Guile itself is needed.
-   (self-native-input? #t)
-
    (native-search-paths
     (list (search-path-specification
            (variable "GUILE_LOAD_PATH")
@@ -141,7 +144,12 @@ without requiring the source code to be rewritten.")
              (base32
               "10lxc6l5alf3lzbs3ihnbfy6dfcrsyf8667wa57f26vf4mk2ai78"))))
    (build-system gnu-build-system)
-   (native-inputs `(("pkgconfig" ,pkg-config)))
+
+   ;; When cross-compiling, a native version of Guile itself is needed.
+   (native-inputs `(,@(if (%current-target-system)
+                          `(("self" ,this-package))
+                          '())
+                    ("pkgconfig" ,pkg-config)))
    (inputs `(("libffi" ,libffi)
              ,@(libiconv-if-needed)
 
@@ -165,8 +173,6 @@ without requiring the source code to be rewritten.")
       ("bdw-gc" ,libgc)
       ("gmp" ,gmp)))
 
-   (self-native-input? #t)
-
    (outputs '("out" "debug"))
 
    (arguments

gnu/packages/ncurses.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2015, 2017, 2018 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015, 2017, 2018, 2019 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2014, 2016 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2015, 2017 Leo Famulari <leo@famulari.name>
 ;;; Copyright © 2016 ng0 <ng0@n0.is>
@@ -196,9 +196,11 @@
                       ,patch-makefile-phase)
                     (add-after 'unpack 'remove-unneeded-shebang
                       ,remove-shebang-phase)))))
-    (self-native-input? #t)           ; for `tic'
     (native-inputs
-     `(("pkg-config" ,pkg-config)))
+     `(,@(if (%current-target-system)
+             `(("self" ,this-package))            ;for `tic'
+             '())
+       ("pkg-config" ,pkg-config)))
     (native-search-paths
      (list (search-path-specification
             (variable "TERMINFO_DIRS")

gnu/packages/python-xyz.scm

@@ -6875,7 +6875,9 @@ Python's @code{ctypes} foreign function interface (FFI).")
                            (string-append "'" file "/lib/libmagic.so'")))
                         #t))))))
     (inputs `(("file" ,file)))
-    (self-native-input? #f)
+    (native-inputs (if (%current-target-system)
+                       `(("self" ,this-package))
+                       '()))
     (synopsis "Python bindings to the libmagic file type guesser.  Note that
 this module and the python-magic module both provide a \"magic.py\" file;
 these two modules, which are different and were developed separately, both