CMAKE: fixed commented out line for later usage
CMAKE: fixed commented out line for later usage

Currently libserialport is cloned and build only once within the binary
directory.
The only way to recompile the lib is to wipe the whole binary directory and
call cmake again. This is sufficient as long as there is no need to modify
the code of libserialport.

fixes #2720

stackforce/serial-interface-cli,stackforce/serial-interface-cli