Accept udev and dracut paths specified by ./configure

There are two common locations where udev and dracut components are
commonly installed.  When building packages using the 'make rpm|deb'
targets check those common locations and pass them to rpmbuild.  For
non-standard configurations these values can be provided by the
the following configure options:

  --with-udevdir=DIR      install udev helpers [default=check]
  --with-udevruledir=DIR  install udev rules [[UDEVDIR/rules.d]]
  --with-dracutdir=DIR    install dracut helpers [default=check]

When rebuilding using the source packages the per-distribution
default values specified in the spec file will be used.  This is
the preferred way to build packages for a distribution but the
ability to override the defaults is provided as a convenience.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2310
Closes #1680

config/user-dracut.m4

@@ -1,8 +1,22 @@
 AC_DEFUN([ZFS_AC_CONFIG_USER_DRACUT], [
-	AC_ARG_WITH(dracutdir,
+	AC_MSG_CHECKING(for dracut directory)
+	AC_ARG_WITH([dracutdir],
 		AC_HELP_STRING([--with-dracutdir=DIR],
-		[install dracut helpers [[EPREFIX/lib/dracut]]]),
-		dracutdir=$withval, dracutdir='${exec_prefix}/lib/dracut')
+		[install dracut helpers @<:@default=check@:>@]),
+		[dracutdir=$withval],
+		[dracutdir=check])
+
+	AS_IF([test "x$dracutdir" = xcheck], [
+		path1=/usr/share/dracut
+		path2=/usr/lib/dracut
+		default=$path2
+
+		AS_IF([test -d "$path1"], [dracutdir="$path1"], [
+			AS_IF([test -d "$path2"], [dracutdir="$path2"],
+				[dracutdir="$default"])
+		])
+	])
 
 	AC_SUBST(dracutdir)
+	AC_MSG_RESULT([$dracutdir])
 ])