Ignore failures on docs generation

[Evgeny Posenitskiy]

We should not require the make all to depend on building the html documentation files using htmlize.el. The reason is that it happens every now and then that the htmlize.el is not present and so each make attempt will try to download it (e.g. after make maintainer-clean). Several issues that I observed:

1. No internet connection at build time: this is quite common (maybe remove htmlize.el from the files to be cleaned?)
2. No access to git: in principal, it would be nice to decouple the build process from the git commands, see one of my last commits #85a0d485575e55b0a04a192331f37538e0ab5900 . For example. building Guix package from the GitHub repo fails exactly because of that (internal issue to clone the git submodule).

[Anthony Scemama]

./configure --disable-doc fixes that problem.