gnu: taglib: 'taglib-config --libs' shows -L flag for zlib.

Fixes compilation of emacs-emms-print-metadata.

* gnu/packages/mp3.scm (taglib)[arguments]: Add #:phases.

gnu/packages/mp3.scm

@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2013 Andreas Enge <andreas@enge.fr>
-;;; Copyright © 2014, 2015, 2017 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015, 2017, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2015 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2016 Efraim Flashner <efraim@flashner.co.il>
 ;;; Copyright © 2017 Thomas Danckaert <post@thomasdanckaert.be>
@@ -174,7 +174,18 @@ a highly stable and efficient implementation.")
     (build-system cmake-build-system)
     (arguments
       '(#:tests? #f ; Tests are not ran with BUILD_SHARED_LIBS on.
-        #:configure-flags (list "-DBUILD_SHARED_LIBS=ON")))
+        #:configure-flags (list "-DBUILD_SHARED_LIBS=ON")
+        #:phases (modify-phases %standard-phases
+                   (add-before 'configure 'adjust-zlib-ldflags
+                     (lambda* (#:key inputs #:allow-other-keys)
+                       ;; Make sure users of 'taglib-config --libs' get the -L
+                       ;; flag for zlib.
+                       (substitute* "CMakeLists.txt"
+                         (("set\\(ZLIB_LIBRARIES_FLAGS -lz\\)")
+                          (string-append "set(ZLIB_LIBRARIES_FLAGS -L"
+                                         (assoc-ref inputs "zlib")
+                                         " -lz)")))
+                       #t)))))
     (inputs `(("zlib" ,zlib)))
     (home-page "https://taglib.org")
     (synopsis "Library to access audio file meta-data")