Only build devname2devid when libudev headers are available

Accidentally introduced by commit 39fc0cb.  The devname2devid utility
which depends on libudev must only be built when libudev headers are
available.  This is accomplished through an AM_CONDITIONAL.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #4416

config/user-libudev.m4

@@ -4,10 +4,17 @@ dnl #
 AC_DEFUN([ZFS_AC_CONFIG_USER_LIBUDEV], [
 	LIBUDEV=
 
-	AC_CHECK_HEADER([libudev.h], [AC_SUBST([LIBUDEV], ["-ludev"])
-	    AC_DEFINE([HAVE_LIBUDEV], 1, [Define if you have libudev])], [])
+	AC_CHECK_HEADER([libudev.h], [
+	    libudev=yes
+	    AC_SUBST([LIBUDEV], ["-ludev"])
+	    AC_DEFINE([HAVE_LIBUDEV], 1, [Define if you have libudev])
+	], [
+	    libudev=no
+	])
 
-	AC_SEARCH_LIBS(i[udev_device_get_is_initialized], [udev], [
+	AM_CONDITIONAL([WANT_DEVNAME2DEVID], [ test x$libudev = xyes ])
+	AC_SEARCH_LIBS([udev_device_get_is_initialized], [udev], [
 	    AC_DEFINE([HAVE_LIBUDEV_UDEV_DEVICE_GET_IS_INITIALIZED], 1, [
 	    Define if udev_device_get_is_initialized is available])], [])
+
 ])

tests/zfs-tests/cmd/devname2devid/Makefile.am

@@ -2,6 +2,8 @@ include $(top_srcdir)/config/Rules.am
 
 pkgexecdir = $(datadir)/@PACKAGE@/zfs-tests/bin
 
+if WANT_DEVNAME2DEVID
 pkgexec_PROGRAMS = devname2devid
 devname2devid_SOURCES = devname2devid.c
 devname2devid_LDADD = -ludev
+endif