libxl: only include utmp.h if it's present

Add a configure check for utmp.h presence, and gate the usage of
utmp.h in libxl to the result of the test.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- resolved minor conflict in configure.ac and reran autogen ]

tools/config.h.in

@@ -51,6 +51,9 @@
 /* Define to 1 if you have the <unistd.h> header file. */
 #undef HAVE_UNISTD_H
 
+/* Define to 1 if you have the <utmp.h> header file. */
+#undef HAVE_UTMP_H
+
 /* Define to 1 if you have the <valgrind/memcheck.h> header file. */
 #undef HAVE_VALGRIND_MEMCHECK_H