BUG of make in Xcode's CLT

Change the environment variable `PATH' in Makefile does NOT effect with make in CLT, and it's OK with the make util which I compiled from original source.

The Makefile

PATH := $(PATH):/opt/bin

export PATH

all:
          @cscope --version

My test

/tmp $ echo $PATH

/usr/bin:/bin:/usr/sbin:/sbin

/tmp $ ls /opt/bin/cscope

/opt/bin/cscope

/tmp $ which make

/usr/bin/make

/tmp $ make

make: cscope: No such file or directory

make: *** [all] Error 1

/tmp $ ./_install/bin/make

cscope: version 15.7a

/tmp $ make --version

GNU Make 3.81

Copyright (C) 2006 Free Software Foundation, Inc.

This is free software; see the source for copying conditions.

There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A

PARTICULAR PURPOSE.

This program built for i386-apple-darwin11.3.0

/tmp $ ./_install/bin/make --version

GNU Make 3.82

Built for x86_64-apple-darwin12.0.0

Copyright (C) 2010 Free Software Foundation, Inc.

License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software: you are free to change and redistribute it.

There is NO WARRANTY, to the extent permitted by law.