Make directory into which mount.zfs is installed configurable

Installing outside of the prefix is not permissible under Gentoo Prefix.
The package manager will cause the installation process to fail if/when
it sees this. I could script a workaround inside the ebuild, but it
seemed to make more sense to make this more configurable.

Signed-off-by: Richard Yao <richard.yao@clusterhq.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2641

cmd/mount_zfs/Makefile.am

@@ -8,7 +8,7 @@ DEFAULT_INCLUDES += \
 # Ignore the prefix for the mount helper.  It must be installed in /sbin/
 # because this path is hardcoded in the mount(8) for security reasons.
 #
-sbindir=/sbin
+sbindir=$(mounthelperdir)
 sbin_PROGRAMS = mount.zfs
 
 mount_zfs_SOURCES = \

config/mount-helper.m4

@@ -0,0 +1,8 @@
+AC_DEFUN([ZFS_AC_CONFIG_USER_MOUNT_HELPER], [
+	AC_ARG_WITH(mounthelperdir,
+		AC_HELP_STRING([--with-mounthelperdir=DIR],
+		[install mount.zfs in dir [[/sbin]]]),
+		mounthelperdir=$withval,mounthelperdir=$sbindir)
+
+	AC_SUBST(mounthelperdir)
+])

config/user.m4

@@ -2,6 +2,7 @@ dnl #
 dnl # Default ZFS user configuration
 dnl #
 AC_DEFUN([ZFS_AC_CONFIG_USER], [
+	ZFS_AC_CONFIG_USER_MOUNT_HELPER
 	ZFS_AC_CONFIG_USER_UDEV
 	ZFS_AC_CONFIG_USER_SYSTEMD
 	ZFS_AC_CONFIG_USER_SYSVINIT