Use env var for sed

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Andrew Innes <andrew.c12@gmail.com>
Closes openzfs#15470

configure.ac

@@ -44,7 +44,7 @@ AM_INIT_AUTOMAKE([subdir-objects foreign])
 # Remove default macros from config.h:
 #   PACKAGE, PACKAGE_{BUGREPORT,NAME,STRING,TARNAME,VERSION}, STDC_HEADERS, VERSION
 AC_CONFIG_HEADERS([zfs_config.h], [
-	sed -nri~ -e '/^$/be' -e 'N;N;/#define (PACKAGE|VERSION|STDC_HEADERS)/d' -e ':e' -e 'p' zfs_config.h && rm zfs_config.h~ || exit])
+	$SED -nri~ -e '/^$/be' -e 'N;N;/#define (PACKAGE|VERSION|STDC_HEADERS)/d' -e ':e' -e 'p' zfs_config.h && rm zfs_config.h~ || exit])
 
 LT_INIT
 AC_PROG_INSTALL