What does CC?= in a Makefile mean?

From http://www.gnu.org/software/make/manual/make.html:

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
defined. This statement:

 FOO ?= bar

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

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

Probably CC is already defined as gcc, so CC ?= g++ won’t override the existing gcc.