Difference between CPPFLAGS and CXXFLAGS in GNU Make

Lazy Set

VARIABLE = value

Normal setting of a variable, but any other variables mentioned with the value field are recursively expanded with their value at the point at which the variable is used, not the one it had when it was declared

Immediate Set

VARIABLE := value

Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time.

Lazy Set If Absent

VARIABLE ?= value

Setting of a variable only if it doesn't have a value. value is always evaluated when VARIABLE is accessed. It is equivalent to

ifeq ($(origin VARIABLE), undefined)
  VARIABLE = value
endif

See the documentation for more details.

Append

VARIABLE += value

Appending the supplied value to the existing value (or setting to that value if the variable didn't exist)

'gmake' refers specifically to GNU make. 'make' refers to the system's default make implementation; on most Linux distros this is GNU make, but on other unixes, it could refer to some other implementation of make, such as BSD make, or the make implementations of various commercial unixes.

The language accepted by GNU make is a superset of the one supported by the traditional make utility.

By using 'gmake' specifically you can use GNU make extensions without worrying about them being misinterpreted by some other make implementation.