In kdump's mkioctls script, use '${CPP}' instead of hardcodedly using

'gcc -E'.  This fixes building when WITH_CLANG_IS_CC is in effect.

Report by:	Niclas Zeising <zeising@daemonic.se>
MFC after:	1 week

usr.bin/kdump/Makefile

@@ -21,7 +21,7 @@ NO_WERROR?=	YES
 CLEANFILES=	ioctl.c kdump_subr.c kdump_subr.h linux_syscalls.c
 
 ioctl.c: mkioctls
-	env MACHINE=${MACHINE} \
+	env MACHINE=${MACHINE} CPP="${CPP}" \
 	    sh ${.CURDIR}/mkioctls print ${DESTDIR}/usr/include > ${.TARGET}
 
 kdump_subr.h: mksubr

usr.bin/kdump/mkioctls

@@ -38,7 +38,7 @@ case "${MACHINE}" in
 esac
 
 awk -v x="$ioctl_includes" 'BEGIN {print x}' |
-	gcc -E -I$1 -dM -DCOMPAT_43TTY - |
+	$CPP -I$1 -dM -DCOMPAT_43TTY - |
 	awk -v ioctl_includes="$ioctl_includes" -v style="$style" '
 BEGIN {
 	print "/* XXX obnoxious prerequisites. */"