Daniel Polanco

Here is a somewhat cleaned up version of TWXS's code:
https://gist.github.com/DanTheColoradan/22bf1ca6307b8edcb53e

if (NOT INSTALLING)
  set (WORKING DIRECTORY "${CMAKECURRENTBINARYDIR}/Install")
  file (MAKEDIRECTORY "${DIRECTORY}")
  executeprocess (COMMAND "${CMAKECOMMAND}"
    -DINSTALLING=TRUE
    -DINSTALLPREFIX=${MYINSTALLPREFIX} "${CMAKECURRENTSOURCEDIR}"
    WORKING_DIRECTORY "${DIRECTORY}"
  )
  executeprocess (COMMAND "${CMAKECOMMAND}"
    --build .
    --config Release WORKING_DIRECTORY "${DIRECTORY}"
  )
else (NOT INSTALLING)
  project ("")
  include (ExternalProject)
  ExternalProjectAdd (externalproject GITREPOSITORY "<gitrepourl>"
    INSTALLDIR "${INSTALLPREFIX}"
    CMAKEARGS -DCMAKEINSTALLPREFIX:PATH=
  )
endif (NOT INSTALLING)