Detect if sed supports --in-place

Not all versions of sed have the --in-place flag. Detect support for
the flag during ./configure and provide a fallback mechanism for those
systems where sed's behavior differs. The autoconf variable
${ac_inplace} can be used to choose the correct flags for editing a
file in place with sed.

Replace violating usages in Makefile.am with ${ac_inplace}.

Reviewed-by: Chris Dunlop <chris@onthe.net.au>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9463

cmd/dbufstat/Makefile.am

@@ -8,6 +8,6 @@ dist_bin_SCRIPTS = dbufstat
 #
 if USING_PYTHON_2
 install-exec-hook:
-	sed --in-place 's|^#!/usr/bin/env python3|#!/usr/bin/env python2|' \
+	sed ${ac_inplace} -e 's|^#!/usr/bin/env python3|#!/usr/bin/env python2|' \
 	    $(DESTDIR)$(bindir)/dbufstat
 endif