[gnome-user-share] build: Check for systemd pkg-config file

[Bastien Nocera <hadess src gnome org>]

commit 05df084a18a7f7b520aedf24be931d590d056e62
Author: Bastien Nocera <hadess hadess net>
Date:   Tue Aug 30 12:40:21 2016 +0200

    build: Check for systemd pkg-config file
    
    So that we don't end up with an empty string at the target directory.

 configure.ac |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 57b3d52..c598fe5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -99,6 +99,7 @@ AM_GLIB_GNU_GETTEXT
 
 dnl ==========================================================================
 dnl systemd user unit directory
+PKG_CHECK_EXISTS (systemd,, AC_MSG_ERROR ([systemd development headers not found]))
 AC_ARG_WITH([systemduserunitdir],
            AS_HELP_STRING([--with-systemduserunitdir=DIR], [Directory for systemd user service files]),
            [],