Problem with "?=" in gmake 3.76.1

[Richard . Phillips]

I was trying to use a gmake feature I had not tried before, the ?=
assignment operator.  The following is a small makefile I constructed to
demonstrate my problem.  Based on my reading of
http://www.gnu.org/manual/make-3.77/html_mono/make.html, the ?= assignment
below should assign "DFLT DFLT DFLT" to MY_FLAGS since it is empty.  

    MY_FLAGS_DFLT := DLFT DFLT DFLT

    MY_FLAGS ?= ${MY_FLAGS_DFLT}

    all:: 
            @ echo "MY_FLAGS: ${MY_FLAGS}"

When I run:

    gmake -f tst all

I get:

    MY_FLAGS: 

Not what I expected.

What don't I understand?????

Thanks in advance
Rick Phillips

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