singular "makes" definitely sounds better to me. I think this is because "what" is closest to the verb and is perceived as singular. However "make" is also acceptable.
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| But I was n't ready for her till then, and what makes me so happy now is to know that I have come to my present way |
... 'But what makes me happy is that now people will be ...http://select.nytimes.com/gst/abstract.html?res=F70C14F83C540C7A8EDDA90994D8404482
October 29, 2000 - By SUSAN SAULNY (NYT) - New York and Region - News - 873 words