Build unit tests only when --enable-unittests is passed

[sergioprado]

Building unit tests by default requires all dependencies to be installed (jpeg, tiff, etc) and users may not want that.

So let's disable the build of unit tests by default and introduce --enable-unittests configure option to enable it.

Signed-off-by: Sergio Prado [email protected]

[dnfadmin]

All CLA requirements met.