mcabber - fixes for autogen.sh

[2bits]

Define a couple of variables to help --HEAD run autogen.sh
and locate our m4 macros. Adds a Dir.chdir for HEAD, and
a depend_on libotr, otherwise configure will choke once you
generate it and run it. Adds a caveat to inform the user
how to setup their .mcabberrc configuration file. This
compiles on Lion and Snow Lep with all five compilers,
either stable or head.

This is a fix for #10048

[jacknagel]

Should this still be conditional?

[jacknagel]

That is, --enable-otr is still listed as an option.

[2bits]

If the user builds head, libotr is required even if they --disable-otr, otherwise there's a configure error.
I didn't feel like making an ugly conditional, as Mcabber is the only program that uses libotr,
and its compile time is 7 seconds. This way it also shows up in brew uses libotr too.

[jacknagel]

Then as long as we're going to install it anyway, should we just be passing --enable-otr to configure by default?

[2bits]

Yes I think we should throw --enable-otr by default. After reading the mcabberrc.example file, I found
that after you --enable-otr, you still have to turn it on inside .mcabberrc. So we would be building a more
featured mcabber, but we would not be imposing otr. I will make another commit.

[2bits]

Let me know if that looks ok @jacknagel. I incorporated changes to make --enable-otr the default behavior.

[jacknagel]

Looks good, pulling.