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[bug #56206] ifndef false positive for exported variable

From: Paul D. Smith
Subject: [bug #56206] ifndef false positive for exported variable
Date: Sun, 12 May 2019 18:43:43 -0400 (EDT)
User-agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:66.0) Gecko/20100101 Firefox/66.0

Follow-up Comment #2, bug #56206 (project make):

Somehow my previous comment was truncated (I do so HATE Savannah markup :( ):

There is another assignment operator for variables, ‘?=’. This is called a
conditional variable
assignment operator, because it only has an effect if the variable is not yet
This statement:

  FOO ?= bar

is exactly equivalent to this (see The origin Function):

  ifeq ($(origin FOO), undefined)
    FOO = bar

Note that a variable set to an empty value is still defined, so ‘?=’ will
not set that variable. 


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