Fix Makefile: doc and clean targets failed to build on Ubuntu.
Fix Makefile: doc and clean targets failed to build on Ubuntu.

DTL-FAIRData/ODEX-FAIRDataPoint,DTL-FAIRData/ODEX-FAIRDataPoint,NLeSC/ODEX-FAIRDataPoint,DTL-FAIRData/ODEX-FAIRDataPoint,NLeSC/ODEX-FAIRDataPoint,NLeSC/ODEX-FAIRDataPoint,NLeSC/ODEX-FAIRDataPoint,DTL-FAIRData/ODEX-FAIRDataPoint