gnu: elogind: Add '-L' flag for libcap in libelogind.la.

This fix allows us to revert this series of commits:

  1b97e9261 * gnu: gdm: Add missing input.
  de0c89f18 * gnu: colord: Add missing input.
  7e38ba654 * gnu: accountsservice: Add missing input.
  166bc1b25 * gnu: network-manager: Add missing input.
  daae20894 * gnu: modem-manager: Add missing input.
  1f919459e * gnu: udisks: Add missing input.
  744e9d074 * gnu: mutter: Add missing input.
  0cce140dc * gnu: gnome-session: Add missing input.
  77c2476ae * gnu: polkit: Add missing input.
  124ac301c * gnu: weston: Add missing input.
  472c2223c * gnu: kmscon: Add missing input.

* gnu/packages/freedesktop.scm (elogind)[arguments]: Add
'add-libcap-to-search-path' phase.
(weston)[inputs]: Remove LIBCAP.
(udisks): Likewise.
(accountsservice): Likewise.
(modem-manager): Likewise.
* gnu/packages/gnome.scm (colord): Likewise
(gnome-session): Likewise.
(mutter): Likewise.
(network-manager): Likewise.
(gdm): Likewise.
* gnu/packages/polkit.scm (polkit): Likewise.
* gnu/packages/terminals.scm (kmscon): Likewise.

gnu/packages/freedesktop.scm

@@ -252,7 +252,17 @@ (define-public elogind
              (substitute* "src/login/org.freedesktop.login1.service"
                (("^Exec=.*")
                 (string-append "Exec=" (assoc-ref %outputs "out")
-                               "/libexec/elogind/elogind\n"))))))))
+                               "/libexec/elogind/elogind\n")))))
+         (add-after 'install 'add-libcap-to-search-path
+           (lambda* (#:key inputs outputs #:allow-other-keys)
+             ;; Add a missing '-L' for libcap in libelogind.la.  See
+             ;; <https://lists.gnu.org/archive/html/guix-devel/2017-09/msg00084.html>.
+             (let ((libcap (assoc-ref inputs "libcap"))
+                   (out    (assoc-ref outputs "out")))
+               (substitute* (string-append out "/lib/libelogind.la")
+                 (("-lcap")
+                  (string-append "-L" libcap "/lib -lcap")))
+               #t))))))
     (native-inputs
      `(("autoconf" ,autoconf)
        ("automake" ,automake)
@@ -451,7 +461,6 @@ (define-public weston
      `(("cairo" ,cairo-xcb)
        ("dbus" ,dbus)
        ("elogind" ,elogind)
-       ("libcap" ,libcap)
        ("libinput" ,libinput-minimal)
        ("libunwind" ,libunwind)
        ("libxcursor" ,libxcursor)
@@ -583,7 +592,6 @@ (define-public udisks
      `(("acl" ,acl)
        ("libatasmart" ,libatasmart)
        ("libgudev" ,libgudev)
-       ("libcap" ,libcap)
        ("polkit" ,polkit)
        ("util-linux" ,util-linux)))
     (outputs '("out"
@@ -684,8 +692,7 @@ (define-public accountsservice
        ("intltool" ,intltool)
        ("pkg-config" ,pkg-config)))
     (inputs
-     `(("libcap" ,libcap)
-       ("shadow" ,shadow)
+     `(("shadow" ,shadow)
        ("polkit" ,polkit)))
     (home-page "http://www.freedesktop.org/wiki/Software/AccountsService/")
     (synopsis "D-Bus interface for user account query and manipulation")
@@ -782,8 +789,7 @@ (define-public modem-manager
     (propagated-inputs
      `(("glib" ,glib))) ; required by mm-glib.pc
     (inputs
-     `(("libcap" ,libcap)
-       ("libgudev" ,libgudev)
+     `(("libgudev" ,libgudev)
        ("libmbim" ,libmbim)
        ("libqmi" ,libqmi)
        ("polkit" ,polkit)))

gnu/packages/gnome.scm

@@ -2781,7 +2781,6 @@ (define-public colord
        ("lcms" ,lcms)))
     (inputs
      `(("dbus-glib" ,dbus-glib)
-       ("libcap" ,libcap)
        ("libgudev" ,libgudev)
        ("libusb" ,libusb)
        ("sqlite" ,sqlite)
@@ -4503,7 +4502,6 @@ (define-public gnome-session
        ("gsettings-desktop-schemas" ,gsettings-desktop-schemas)
        ("gtk+" ,gtk+)
        ("json-glib" ,json-glib)
-       ("libcap" ,libcap)
        ("libsm" ,libsm)
        ("libxcomposite" ,libxcomposite)
        ("libxtst" ,libxtst)
@@ -4739,7 +4737,6 @@ (define-public mutter
      `(("elogind" ,elogind)
        ("gnome-desktop" ,gnome-desktop)
        ("libcanberra-gtk" ,libcanberra)
-       ("libcap" ,libcap)
        ("libgudev" ,libgudev)
        ("libice" ,libice)
        ("libsm" ,libsm)
@@ -5023,7 +5020,6 @@ (define-public network-manager
        ("iptables" ,iptables)
        ("isc-dhcp" ,isc-dhcp)
        ("jansson" ,jansson)
-       ("libcap" ,libcap)
        ("libgcrypt" ,libgcrypt)
        ("libgudev" ,libgudev)
        ("libndp" ,libndp)
@@ -5254,7 +5250,6 @@ (define-public gdm
        ("gtk+" ,gtk+)
        ("iso-codes" ,iso-codes)
        ("libcanberra" ,libcanberra)
-       ("libcap" ,libcap)
        ("linux-pam" ,linux-pam)))
     (synopsis "Display manager for GNOME")
     (home-page "https://wiki.gnome.org/Projects/GDM/")

gnu/packages/polkit.scm

@@ -81,7 +81,6 @@ (define-public polkit
     (build-system gnu-build-system)
     (inputs
      `(("expat" ,expat)
-       ("libcap" ,libcap)
        ("linux-pam" ,linux-pam)
        ("elogind" ,elogind)
        ("mozjs" ,mozjs)

gnu/packages/terminals.scm

@@ -3,7 +3,7 @@
 ;;; Copyright © 2016 Mckinley Olsen <mck.olsen@gmail.com>
 ;;; Copyright © 2016, 2017 Alex Griffin <a@ajgrf.com>
 ;;; Copyright © 2016 David Craven <david@craven.ch>
-;;; Copyright © 2016 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2016, 2017 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2016, 2017 José Miguel Sánchez García <jmi2k@openmailbox.org>
 ;;; Copyright © 2017 Tobias Geerinckx-Rice <me@tobias.gr>
 ;;; Copyright © 2017 Ricardo Wurmus <rekado@elephly.net>
@@ -239,8 +239,7 @@ (define-public kmscon
        ("libxml2" ,libxml2)                       ;for XML_CATALOG_FILES
        ("docbook-xsl" ,docbook-xsl)))
     (inputs
-     `(("libcap" ,libcap)
-       ("libdrm" ,libdrm)
+     `(("libdrm" ,libdrm)
        ("libtsm" ,libtsm)
        ("libxkbcommon" ,libxkbcommon)
        ("logind" ,elogind)