mk: Add mktool support to distinfo.

mktool is an optional external tool, available from pkgtools/mktool or
"cargo install mktool", that provides significant performance improvements
compared to distinfo.awk, while being 100% compatible.

After installing, set TOOLS_PLATFORM.mktool=/path/to/mktool to enable it.

Comparing the time taken to run "bmake distinfo" on a SmartOS host inside
www/grafana, distinfo.awk takes:

  real	3m37.603s
  user	1m16.887s
  sys	1m53.369s

while mktool takes:

  real	0m12.516s
  user	0m10.314s
  sys	0m8.485s

It should be noted that the vast majority of that time is in the generation
of the distinfo input file.  mktool itself takes around 2 seconds.  Patches
to improve input file generation are available, but are dependent on bmake
with temporary file support due to argmax limits on NetBSD.

mk/checksum/checksum.mk

@@ -1,4 +1,4 @@
-# $NetBSD: checksum.mk,v 1.31 2024/10/11 11:20:23 jperkin Exp $
+# $NetBSD: checksum.mk,v 1.32 2024/10/11 11:40:29 jperkin Exp $
 #
 # See bsd.checksum.mk for helpful comments.
 #
@@ -53,9 +53,13 @@ checksum checksum-phase:
 	fi
 .endif
 
+.if !empty(TOOLS_PLATFORM.mktool)
+_DISTINFO_CMD=	${TOOLS_PLATFORM.mktool} distinfo
+.else
 _DISTINFO_CMD=	${PKGSRC_SETENV} DIGEST=${TOOLS_DIGEST:Q} SED=${TOOLS_SED:Q} \
 			TEST=${TOOLS_TEST:Q} WC=${TOOLS_WC:Q}	\
 		${AWK} -f ${PKGSRCDIR}/mk/checksum/distinfo.awk --
+.endif
 
 .if exists(${DISTDIR})
 _DISTINFO_ARGS_COMMON+=	-d ${DISTDIR}