configure: don't silently disable systemd support

Originally when user runs ./configure --enable-systemd and systemd
development library is not available the build system silently disables
systemd support. This is not in line with normal expectation.

Instead, configure should error out when user has asked for systemd
support but development libraries can't be found.

Reported-by: George Dunlap <george.dunlap@eu.citrix.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

m4/systemd.m4

@@ -85,7 +85,11 @@ AC_DEFUN([AX_CHECK_SYSTEMD], [
 		AC_DEFINE([HAVE_SYSTEMD], [1], [Systemd available and enabled])
 			systemd=y
 			AX_CHECK_SYSTEMD_LIBS()
-	    ],[systemd=n])
+	    ],[
+		AS_IF([test "x$enable_systemd" = "xyes"],
+			[AC_MSG_ERROR([Unable to find systemd development library])],
+			[systemd=n])
+	    ])
 	],[systemd=n])
 ])