meson: make sure html symlinks are also created in build directory

meson: make sure html symlinks are also created in build directory

The symlinks should be created in the build directory in two cases: when
configuration specifies -Dhtml=true, or when ninja html target is built.
Normally install : {true,false} is used to decide if a target should be built,
but in this case, we cannot use install : true, because, as described in
488477d101, that results in the target file being copied into the
installation directory instead of a symlink. So we need a work-around. To
achieve the first end, the commands to create the symlinks are added as
dependencies of the command to create the html page. To the second end, they
are added as dependencies of the html target.

Follow-up for 488477d101 and 064d9ef0d7.