FOO ?= bar

is equivalent to

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

However, when CC is not set its value is 'default' rather than
'undefined'.

>From [1]:

'undefined': if variable was never defined.

'default': if variable has a default definition, as is usual with CC
and so on. See Variables Used by Implicit Rules. Note that if you have
redefined a default variable, the origin function will return the
origin of the later definition.

[1] https://www.gnu.org/software/make/manual/make.htmlSigned-off-by: Radostin Stoyanov <rstoyanov1@gmail.com>
Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>