build: avoid corrupting / in RHEL 5

build: avoid corrupting / in RHEL 5

I noticed this while building from libvirt.git on RHEL 5.6:

Generating internals/command.html.tmp
mkdir: cannot create directory `/internals': Permission denied

If I had been building as root instead, this pollutes /.

Older autoconf lacks $(builddir), but it is rigorously equal to '.'
in newer autoconf, so we could use '$(MKDIR_P) internals' instead.

However, since internals/command.html is part of the tarball, we
_already_ build it in $(srcdir), not $(builddir) during VPATH
builds, so the mkdir is wasted effort!

* docs/Makefile.am (internals/%.html.tmp): Drop unused mkdir.