Build: Only run Doxygen once
Build: Only run Doxygen once

Before this change a parallel build started multiple instances of
`doxygen` concurrently. Each instance of Doxygen invokes the Markdown
Link Converter as filter program. Some of the Markdown Link Converter
processes accessed and wrote the same files concurrently. This would
cause some of them to fail, since each Markdown Link Converter assumes,
that it has exclusive access to its input file.

This commit fixes #686.

petermax2/libelektra,ElektraInitiative/libelektra,BernhardDenner/libelektra,mpranj/libelektra,BernhardDenner/libelektra,e1528532/libelektra,BernhardDenner/libelektra,BernhardDenner/libelektra,ElektraInitiative/libelektra,petermax2/libelektra,mpranj/libelektra,mpranj/libelektra,ElektraInitiative/libelektra,mpranj/libelektra,petermax2/libelektra,mpranj/libelektra,mpranj/libelektra,ElektraInitiative/libelektra,mpranj/libelektra,e1528532/libelektra,petermax2/libelektra,mpranj/libelektra,e1528532/libelektra,petermax2/libelektra,petermax2/libelektra,mpranj/libelektra,e1528532/libelektra,e1528532/libelektra,petermax2/libelektra,e1528532/libelektra,BernhardDenner/libelektra,e1528532/libelektra,petermax2/libelektra,ElektraInitiative/libelektra,e1528532/libelektra,petermax2/libelektra,ElektraInitiative/libelektra,ElektraInitiative/libelektra,ElektraInitiative/libelektra,BernhardDenner/libelektra,ElektraInitiative/libelektra,mpranj/libelektra,ElektraInitiative/libelektra,BernhardDenner/libelektra,BernhardDenner/libelektra,ElektraInitiative/libelektra,BernhardDenner/libelektra,mpranj/libelektra