Don't rebuild ioctl.c and relink libsysdecode if there are no changes

Instead generate a temporary file and only overwrite ioctl.c if the
files are actually different.

Approved By:	jhb (mentor)

lib/libsysdecode/Makefile

@@ -129,12 +129,17 @@ tables.h: mktables
 # mkioctls runs find(1) for headers so needs to rebuild every time.  This used
 # to be a hack only done in buildworld.
 .if !defined(_SKIP_BUILD)
-ioctl.c: .PHONY
+ioctl.c.tmp: .PHONY
 .endif
-ioctl.c: mkioctls .META
+ioctl.c.tmp: mkioctls .META
 	env CPP="${CPP}" MK_PF="${MK_PF}" \
 	    /bin/sh ${.CURDIR}/mkioctls ${SYSROOT:U${DESTDIR}}${INCLUDEDIR} > ${.TARGET}
 
+ioctl.c: ioctl.c.tmp
+	if [ ! -e ${.TARGET} ] || ! cmp -s ${.TARGET} ${.TARGET}.tmp; then \
+		mv -f ${.TARGET}.tmp ${.TARGET}; \
+	fi
+
 beforedepend: ioctl.c tables.h
 
 .include <bsd.lib.mk>