gnu: libsigsegv: Install the shared library.

Suggested by Weldon Goree <weldon@b.rontosaur.us>.

* gnu/packages/libsigsegv.scm (libsigsegv)[arguments]: Add
  #:configure-flags.

gnu/packages/libsigsegv.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2018 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2017 Efraim Flashner <efraim@flashner.co.il>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -38,12 +38,15 @@
    (home-page "https://www.gnu.org/software/libsigsegv/")
    (synopsis "Library for handling page faults")
    (arguments
+    `(;; The shared library isn't built by default but some packages need it.
+      #:configure-flags '("--enable-shared")
+
       ;; On MIPS, work around this error:
       ;;
       ;; In file included from fault-linux-mips-old.h:18:0,
       ;;    [...]
       ;; linux-libre-headers-cross-mips64el-linux-gnu-3.3.8/include/asm/sigcontext.h:57:8: error: redefinition of 'struct sigcontext'
-    (if (string-contains (or (%current-target-system) (%current-system))
+      ,@(if (string-contains (or (%current-target-system) (%current-system))
                              "mips64el")
             `(#:phases (modify-phases %standard-phases
                          (add-before 'configure 'patch-mips-old-h
@@ -51,7 +54,7 @@
                              (substitute* "src/fault-linux-mips-old.h"
                                (("#include <asm/sigcontext\\.h>") ""))
                              #t))))
-        '()))
+            '())))
    (description
     "GNU libsigsegv is a library to handle page faults, which occur when a
 program tries to access an unavailable region of memory, in user mode.  By