makefile := = 차이점

 In short, variables defined with := are expanded once, but variables defined with = are expanded whenever they are used.

[링크 : http://stackoverflow.com/.../whats-the-difference-between-and-in-makefile]

Simply expanded variables are defined by lines using ‘:=’ or ‘::=’ (see Setting Variables). Both forms are equivalent in GNU make; however only the ‘::=’ form is described by the POSIX standard (support for ‘::=’ was added to the POSIX standard in 2012, so older versions of make won’t accept this form either).

[링크 : http://www.gnu.org/software/make/manual/html_node/Flavors.html#Flavors]

https://www.gnu.org/software/make/manual/html_node/Setting.html#Setting