ld_library_path does not work in makefile

The makefile has the following commands:

exrtl:
        export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:libs

clean:
        export LD_LIBRARY_PATH=""

But nothing happens after them. If you then enter

echo $LD_LIBRARY_PATH

Then nothing will change. You have to enter it manually. Is there any way to fix this?