doc: allow building htmldoc with Sphinx 3+

Due to removed function c_funcptr_sig_re building with Sphinx 3 fails.

With the patch building succeeds with a lot of warnings if the '-W' flag is
removed from doc/Makefile. Most of the documentation is correct

This follows the approach taken by the Linux kernel.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>