gnu: r-minimal: Work around failure to embed reference to "which".

* gnu/packages/statistics.scm (r-minimal)[arguments]: Add phase "patch-which".
[native-inputs]: Move "which" from here...
[propagated-inputs]: ...to here.

gnu/packages/statistics.scm

@@ -126,6 +126,12 @@ be output in text, PostScript, PDF or HTML.")
              "PKG_BUILT_STAMP=1970-01-01")
        #:phases
        (modify-phases %standard-phases
+         ;; FIXME: see bug #28157.
+         (add-before 'configure 'patch-which
+           (lambda* (#:key inputs #:allow-other-keys)
+             (substitute* "src/library/base/R/unix/system.unix.R"
+               (("@WHICH@") "which"))
+             #t))
          (add-before 'configure 'patch-uname
            (lambda* (#:key inputs #:allow-other-keys)
              (let ((uname-bin (string-append (assoc-ref inputs "coreutils")
@@ -227,7 +233,6 @@ be output in text, PostScript, PDF or HTML.")
        ("perl" ,perl)
        ("pkg-config" ,pkg-config)
        ("texinfo" ,texinfo) ; for building HTML manuals
-       ("which" ,which) ; for tests/Examples/base-Ex.R
        ("tzdata" ,tzdata-2017a)
        ("xz" ,xz)))
     (inputs
@@ -246,6 +251,9 @@ be output in text, PostScript, PDF or HTML.")
        ("pcre" ,pcre)
        ("readline" ,readline)
        ("zlib" ,zlib)))
+    ;; FIXME: By default Sys.which embeds a reference to "which", but this
+    ;; reference is not detected by Guix (see bug #28157).
+    (propagated-inputs `(("which" ,which)))
     (native-search-paths
      (list (search-path-specification
             (variable "R_LIBS_SITE")