As MadScientist pointed out, you can export individual variables with:
export MY_VAR = foo # Available for all targets
Or export variables for a specific target (target-specific variables):
my-target: export MY_VAR_1 = foo
my-target: export MY_VAR_2 = bar
my-target: export MY_VAR_3 = baz
my-target: dependency_1 dependency_2
echo do something
You can also specify the .EXPORT_ALL_VARIABLES target to—you guessed it!—EXPORT ALL THE THINGS!!!:
.EXPORT_ALL_VARIABLES:
MY_VAR_1 = foo
MY_VAR_2 = bar
MY_VAR_3 = baz
test:
@echo $$MY_VAR_1 $$MY_VAR_2 $$MY_VAR_3