doc/Makefile.am: Create directory for pdf output
doc/Makefile.am: Create directory for pdf output

dblatex is passed the option `-o $(srcdir)/pdf`, but if this directory
does not exist, the PDF file is created with the name 'pdf' in
$(srcdir).  Make the directory so that the -o option treats the
specified name as an output directory instead of an output filename.

Linux-Comedi/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,olsonse/comedilib,olsonse/comedilib,olsonse/comedilib,olsonse/comedilib,olsonse/comedilib,olsonse/comedilib,Linux-Comedi/comedilib