config: better libtirpc detection

Improve the autoconf code for finding libtirpc and do not assume the
headers are in /usr/include/tirpc.

Also remove this assumption from the rpc/xdr.h header in libspl and
use the same #include_next mechanism that is used for other libspl
headers.

Include pkg.m4 from pkg-config in config/ for PKG_CHECK_MODULES(), the
file license allows this.

Include ax_save_flags.m4 and ax_restore_flags.m4 from autoconf-archive,
the file licenses are compatible. Use the 2012 versions so as not rely
on a more recent autoconf feature AS_VAR_COPY(), which breaks some build
slaves.

Add new macro library config/find_system_library.m4 which defines the
FIND_SYSTEM_LIBRARY() macro which is a convenience wrapper over using
PKG_CHECK_MODULES() with a fallback to standard library locations and
some sanity checks.

The parameters are:

FIND_SYSTEM_LIBRARY(VARIABLE-PREFIX, MODULE, HEADER, HEADER-PREFIXES,
    LIBRARY, FUNCTIONS, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])

HEADER-PREFIXES and FUNCTIONS are comma-separated m4 lists.

For libtirpc we are using:

FIND_SYSTEM_LIBRARY(LIBTIRPC, [libtirpc], [rpc/xdr.h], [tirpc], [tirpc],
    [xdrmem_create], [], [...])

The headers are first checked for without the prefixes and then with.

This system works with pkg-config and falls back on checking standard
header/library locations, it can be easily overridden by the user by
setting the PREFIX_CFLAGS and PREFIX_LIBS variables which are
automatically added to the ./configure --help output.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rafael Kitover <rkitover@gmail.com>
Closes #7422
Closes #8313