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

config/find_system_library.m4

@@ -0,0 +1,73 @@
+# find_system_lib.m4 - Macros to search for a system library.   -*- Autoconf -*-
+
+dnl requires pkg.m4 from pkg-config
+dnl requires ax_save_flags.m4 from autoconf-archive
+dnl requires ax_restore_flags.m4 from autoconf-archive
+
+dnl FIND_SYSTEM_LIBRARY(VARIABLE-PREFIX, MODULE, HEADER, HEADER-PREFIXES, LIBRARY, FUNCTIONS, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])
+
+AC_DEFUN([FIND_SYSTEM_LIBRARY], [
+    AC_REQUIRE([PKG_PROG_PKG_CONFIG])
+
+    _library_found=
+
+    PKG_CHECK_MODULES([$1], [$2], [_library_found=1], [
+        AS_IF([test -f /usr/include/[$3]], [
+            AC_SUBST([$1][_CFLAGS], [])
+            AC_SUBST([$1][_LIBS], ["-l[$5]]")
+            _library_found=1
+        ],[ AS_IF([test -f /usr/local/include/[$3]], [
+            AC_SUBST([$1][_CFLAGS], ["-I/usr/local/include"])
+            AC_SUBST([$1][_LIBS], ["-L/usr/local -l[$5]]")
+            _library_found=1
+        ],[dnl ELSE
+            m4_foreach([prefix], [$4], [
+                AS_IF([test "x$_library_found" != "x1"], [
+                    AS_IF([test -f [/usr/include/]prefix[/][$3]], [
+                        AC_SUBST([$1][_CFLAGS], ["[-I/usr/include/]prefix["]])
+                        AC_SUBST([$1][_LIBS], ["-l[$5]]")
+                        _library_found=1
+                    ],[ AS_IF([test -f [/usr/local/include/]prefix[/][$3]], [
+                        AC_SUBST([$1][_CFLAGS], ["[-I/usr/local/include/]prefix["]])
+                        AC_SUBST([$1][_LIBS], ["-L/usr/local -l[$5]"])
+                        _library_found=1
+                    ])])
+                ])
+            ])
+        ])])
+
+        AS_IF([test -z "$_library_found"], [
+            AC_MSG_WARN([cannot find [$2] via pkg-config or in the standard locations])
+        ])
+    ])
+
+    dnl do some further sanity checks
+
+    AS_IF([test -n "$_library_found"], [
+        AX_SAVE_FLAGS
+
+        CPPFLAGS="$CPPFLAGS $(echo $[$1][_CFLAGS] | sed 's/-include */-include-/g; s/^/ /; s/ [^-][^ ]*//g; s/ -[^Ii][^ ]*//g; s/-include-/-include /g; s/^ //;')"
+        CFLAGS="$CFLAGS $[$1][_CFLAGS]"
+        LDFLAGS="$LDFLAGS $[$1][_LIBS]"
+
+        AC_CHECK_HEADER([$3], [], [
+            AC_MSG_WARN([header [$3] for library [$2] is not usable])
+            _library_found=
+        ])
+
+        m4_foreach([func], [$6], [
+            AC_CHECK_LIB([$5], func, [], [
+                AC_MSG_WARN([cannot find ]func[ in library [$5]])
+                _library_found=
+            ])
+        ])
+        
+        AX_RESTORE_FLAGS
+    ])
+
+    AS_IF([test -n "$_library_found"], [
+        :;$7
+    ],[dnl ELSE
+        :;$8
+    ])
+])