gnu: lilv: Set rpath via LDFLAGS.

Fixes <http://bugs.gnu.org/20040>.

* gnu/packages/audio.scm (lilv): Add a pre-configure phase setting $LDFLAGS to
  set the rpath to $out/lib.

gnu/packages/audio.scm

@@ -713,7 +713,16 @@ (define-public lilv
               (base32
                "0aj2plkx56iar8vzjbq2l7hi7sp0ml99m0h44rgwai2x4vqkk2j2"))))
     (build-system waf-build-system)
-    (arguments `(#:tests? #f)) ; no check target
+    (arguments
+     `(#:tests? #f ; no check target
+       #:phases
+       (modify-phases %standard-phases
+         (add-before
+          'configure 'set-ldflags
+          (lambda* (#:key outputs #:allow-other-keys)
+            (setenv "LDFLAGS"
+                    (string-append "-Wl,-rpath="
+                                   (assoc-ref outputs "out") "/lib")))))))
     ;; required by lilv-0.pc
     (propagated-inputs
      `(("serd" ,serd)