It should not be lost on anyone the intent in leaving off the last sentence of the quote: "It would, indeed, be more sensible to build houses and the like; but if there are political and practical difficulties in the way of this, the above would be better than nothing."

That intent being to mislead the reader as to the actual Keynesian argument. In the first part of the quote Keynes is being somewhat facetious in drawing an analogy to essentially unproductive economic activities like gold-mining or waging war which none-the-less can have a stimulative economic effect. His principal conclusion however is that the ideal solution to unemployment is for the government to be the employer of last resort. Identify some productive activity which the private sector is not engaged in and have the government put unemployed people to work doing these productive activities.