Don't hardcode /usr/bin as the path for mktemp in build tools

It won't work e.g. when crossbuilding from Ubuntu Linux as mktemp is in
/bin there.

Reviewed By:	bdrewery
Approved By:	jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D13937

Makefile.inc1

@@ -463,7 +463,7 @@ TMPPATH=	${STRICTTMPPATH}:${PATH}
 # when in the middle of installing over this system.
 #
 .if make(distributeworld) || make(installworld) || make(stageworld)
-INSTALLTMP!=	/usr/bin/mktemp -d -u -t install
+INSTALLTMP!=	mktemp -d -u -t install
 .endif
 
 .if make(stagekernel) || make(distributekernel)

bin/sh/mkbuiltins

@@ -34,7 +34,7 @@
 #	@(#)mkbuiltins	8.2 (Berkeley) 5/4/95
 # $FreeBSD$
 
-temp=`/usr/bin/mktemp -t ka`
+temp=`mktemp -t ka`
 havehist=1
 if [ "X$1" = "X-h" ]; then
 	havehist=0

bin/sh/mktokens

@@ -38,7 +38,7 @@
 # token marks the end of a list.  The third column is the name to print in
 # error messages.
 
-temp=`/usr/bin/mktemp -t ka`
+temp=`mktemp -t ka`
 cat > $temp <<\!
 TEOF	1	end of file
 TNL	0	newline