gnu: wxwidgets: Set rpath in LDFLAGS.

Fixes <http://bugs.gnu.org/20049>.

* gnu/packages/wxwidgets.scm (wxwidgets): Add a make flag to set rpath to
  $out/lib via LDFLAGS.

gnu/packages/wxwidgets.scm

@@ -62,6 +62,9 @@ (define-public wxwidgets
     (arguments
      '(#:configure-flags
        '("--with-regex=sys" "--with-libmspack" "--with-sdl")
+       #:make-flags
+       (list (string-append "LDFLAGS=-Wl,-rpath="
+                            (assoc-ref %outputs "out") "/lib"))
        ;; No 'check' target.
        #:tests? #f))
     (home-page "https://www.wxwidgets.org/")