Protocol "https" not supported or disabled in libcurl

I'm running into this problem on a somewhat dated Fedora 21 system when the build gets to vxl:

[ 29%] Performing download step (download, verify and extract) for 'vxl'
-- downloading...
     src='https://github.com/judajake/vxl/archive/44433e4bd8ca3eabe4e5441444bf2a050d689d45.tar.gz'
     dst='/scratch/cmb-tpl/downloads/44433e4bd8ca3eabe4e5441444bf2a050d689d45.tar.gz'
     timeout='none'
CMake Error at vxl-stamp/download-vxl.cmake:27 (message):
  error: downloading
  'https://github.com/judajake/vxl/archive/44433e4bd8ca3eabe4e5441444bf2a050d689d45.tar.gz'
  failed

    status_code: 1
    status_string: "Unsupported protocol"
    log: Protocol "https" not supported or disabled in libcurl

  Closing connection -1

/usr/bin/curl itself has no problems with https, so I'm puzzled as to what is going on here. Suggestions?