PSEiBlog

While configuring your MTA and additional services on a Gentoo machine, you are likely to follow the handbook at http://www.gentoo.org/doc/en/qmail-howto.xml .
Though this guide is pretty good, at one point you will encounter a rather large annoyance.
Once you’re ready to set up webmail, by means of horde imp, it seems the only thing you have to do is
emerge horde-imp what should do the trick.
Usually this is true, except for the fact that imp heavily relies on a load of PEAR modules. These PEAR modules will be merged prior to merging imp, and this is where all goes to hell.
You probably had PHP preinstalled on the system, and each PEAR module (there’s about 25 of them for imp) will try to use your premerged PHP installation. If you used a default PHP merge, nearly every single one of them will break, because PHP was compiled without a USE flag that the module needs.
So you’ll find yourself adding the USE flag to your /etc/make.conf, re-emering php, and trying to re-emerge horde-imp afterwards, which results in the module installing, and the process breaking on the next module once more, forcing you to repeat the process. This can take you quite a while, since PHP isn’t quite the smallest package and even on a quite powerful server it takes nearly 10 minutes each run.

So.. how can you avoid a solid 3 hour emerge stint? By learning from my trial-and-error-process, and adding all of the needed USE flags to your /etc/make.conf:
pam tcpd ssl mysql apache2 cli pcre xml zlib ftp ctype pcre session imap
afterwards emerge php and emerge horde-imp and you’re good to go.