How to append some text to a variable in makefile?

You need the := in place of the recursive =:

FOO := hello
FOO := $(FOO)_world
$(info FOO=$(FOO))

hello_world

I would say the makefile addsuffix function is what you are looking for, thus in your case for example:

FOO := hello
FOO := $(addsuffix _world,$(FOO))

However @user218374 was right too and his solution works as well, in this case, but I am pretty sure you might be happy to have a look at these built-in expansion functions relating also to lists of file names!