Don't use "mkdir" during install

Don't use "mkdir" during install

lahwran just showed up on irc and told us that he installed znc, but znc failed
to find any modules. The reason for this was his umask 077 which means that
"make install" installed stuff so that only root can access it.

The solution is do use "install -d" since that makes sure to ignore the
currently set umask.

However, google finds results which say that "install -d" might mess with stuff
of pre-existing directories when it shouldn't, so we must first test if the
directory already exists before calling install. Obviously, this makes our
Makefile a lot more readable. :-(

I didn't have time to test this properly, so stuff might break.

Signed-off-by: Uli Schlachter <redacted>
Conflicts:

	modules/Makefile.in