... to create the LaTeX header file used by doxygen.
This fix applies to both configure/make and CMake.
The existing version used `doxygen` which erroneously ran the doxygen
version from the current 'PATH' which might issue doxygen warnings and
potentially create a wrong LaTeX header file if the configured doxygen
program was not the one in the user's path.
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.
