Fixes .configure syntax error on mac os X

The exact error I got was:
./configure: line 5578: syntax error near unexpected token `,'
./configure: line 5578: `LIBCURL_CHECK_CONFIG(, 7.15.2, ,'

Fixed on my machine with help from  https://github.com/Neisklar/quarkcoin-cpuminer/pull/3/files
Merge pull request #4 from jomz/master

Fixes .configure syntax error on mac os X