crossbuild: Make the CHECK_TIME variable work on Linux

Linux /usr/bin/find doesn't understand the -mtime -0s flag.
Instead create a temporary file and compare that file's mtime to
sys/sys/param.h to check whether the clock is correct.

Reviewed By:	jhb, imp
Approved By:	jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D14157

Makefile

@@ -352,7 +352,7 @@ _guard: .PHONY
 	@false
 
 STARTTIME!= LC_ALL=C date
-CHECK_TIME!= find ${.CURDIR}/sys/sys/param.h -mtime -0s ; echo
+CHECK_TIME!= cmp=`mktemp`; find ${.CURDIR}/sys/sys/param.h -newer "$$cmp" && rm "$$cmp"; echo
 .if !empty(CHECK_TIME)
 .error check your date/time: ${STARTTIME}
 .endif