using external_project breaks a uniprocessor build since the libraries
are only partially copied to solver.

Change-Id: I0a83486e466e1c062a1dad89d9ea3d9e78d8c9b1
Reviewed-on: https://gerrit.libreoffice.org/1048Reviewed-by: Peter Foley <jpfoley2@gmail.com>
Reviewed-by: Tor Lillqvist <tml@iki.fi>
Tested-by: Tor Lillqvist <tml@iki.fi>