The LaTeX header file used when creating PDF documentation depends on
the doxygen and latex versions, respectively.
The old header file 'documentation/src/fltk-book.tex.in' had been
generated manually and needed to be updated for new doxygen versions
which made the PDF documentation generation dependent on the
doxygen/latex versions on the build system.
The new LaTeX header file 'fltk-book.tex' is generated by doxygen,
i.e. taking into account the doxygen and latex versions on the build
system and "edited" to include the page title defined in the new
file 'documentation/src/fltk-title.tex.in'.
This makes the PDF documentation independent of the doxygen and
latex versions of the build system.
