Autoconf 2.70 compatibility

Autoconf 2.70 compatibility

Several m4 macros have been retired in autoconf 2.70.  Update the
the build system to use the new macros provided to replace them.

* Replaced AC_HELP_STRING with AS_HELP_STRING.

* Replaced AC_TRY_COMPILE with AC_COMPILE_IFELSE/AC_LANG_PROGRAM.

* Replaced AC_CANONICAL_SYSTEM with AC_CANONICAL_TARGET

* Replaced AC_PROG_LIBTOOL with LT_INIT

* $CPP is not defined in ZFS_AC_KERNEL and really shouldn't be
  directly used like this.  Replace it with an $AWK command
  to extract the kernel source version.

Reviewed-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #11413
Closes #11419