Reenable xz support by default

For https://bugzilla.gnome.org/show_bug.cgi?id=757466

problem was introduced by commit f3f86ff465c92c79f834d7b981f3c7274a8bb5c8
for https://bugzilla.gnome.org/show_bug.cgi?id=711026
diff --git a/configure.ac b/configure.ac
index 14ac0a8..48e0577 100644
--- a/configure.ac
+++ b/configure.ac
@@ -445,6 +445,9 @@
                 fi],
                 [have_liblzma=no])
 	     LDFLAGS="${SAVE_LDFLAGS}"])
+    else
+	# we still need to check for lzma,h header
+	AC_CHECK_HEADERS([lzma.h])
     fi
 
     # Found the library via either method?