Jeffrey Sachs believes the reason why Africans are so poor is because of their unfortunate climate. The country is hot, and this causes problems with malaria, etc. There are also very few coastlines compared to land area, which means most Africans cannot trade with the outside world via ships.
Others believe the problem lies in the lack of institutions, like courts, police, etc. The governments presently in Africa are accused of corruption.
Either way, the libertarian/capitalist ideology provides a solution to the problem. The problem is that the African people are living in an area that is not good. If immigration were completely free then the African people can move to better climates where there may be better institions, e.g. to Europe or to Asia via land or to America via sea. The decrease in the number of people in Africa will lower land prices. Industries will then move into Africa to exploit not only natural resources but also cheap land prices. Africa then will become a new industrial continent, specializing in that area in which it has a comparative advantage, i.e. producing materials.
The essense of libertarianism or capitalism is that there is freedom for economic inputs like labor or capital to move to where it is most productive. Freedom of immigration then is not only an essential Human Right but an important prerequisite for economic efficiency.