configure.in: require yapps executable

[zultron]

Fixes #475; 2.7 counterpart for inttool-extract only in #476

[SebKuzminsky]

The commit message for bf985533a99e1 is wrong (talks about intltool), but the patch is right (checks for yapps). Please fix the commit message and I'll merge it.

[SebKuzminsky]

I merged #476 into 2.7 and merged 2.7 into master, so if you rebase bf98553 onto master you won't need the merge in this PR, simplifying it some.

[zultron]

D'oh! I've been making a bunch of embarrassing copy and paste mistakes lately.

I see that #475 was merged, but I don't see it in master.

Anyway, the PR is fixed. Thanks, @SebKuzminsky.

[SebKuzminsky]

Oops, you're right, I didn't merge #476 into master, it's just in 2.7. Derp. I'll fix it.

[zultron]

Jeezus H. Beezus, @SebKuzminsky you caught the commit with the wrong name, but missed the PR misnamed "configure.in: require yaml2 executable"! I can't use the "copy'n'paste" excuse for that one.

Fixed PR title. :D