Ticket 47968 - When systemd libs not found, error instead of continuing.

Bug Description:  Systemd-devel isn't installed, we still build and weird
things happen.

Fix Description:  Make sure to error if we have --with-systemd and no
systemd-devel package

https://fedorahosted.org/389/ticket/47968

Author: wibrown

Review by: One line patch

m4/systemd.m4

@@ -30,7 +30,7 @@ if test "$with_systemd" = yes; then
         systemd_lib=`$PKG_CONFIG --libs-only-l systemd libsystemd-journal libsystemd-daemon`
         systemd_defs="-DWITH_SYSTEMD"
     else
-        AC_MSG_RESULT([no Systemd pkg-config files])
+        AC_MSG_ERROR([no Systemd pkg-config files])
     fi
 
 fi