Document that installing other software in /usr/local can break ports

[ryandesign@…]

We often field questions on the mailing lists regarding some port that doesn't install with some obscure error, and this is often the result of rogue software installed in /usr/local. readline is a frequent culprit but any software installed there is a bad idea. The Guide should recommend not having anything installed in /usr/local. Note that having things installed in /usr/local/anything is not a problem at all. For example, having /usr/local/mysql is fine. However having things directly in /usr/local is a problem because the compiler looks there by default and will find libraries that are installed there. So having things in /usr/local/bin and /usr/local/lib for example are problematic and not supported. Users who experience problems and who have the directory /usr/local are advised as a troubleshooting step to rename it (e.g. to /usr/local-off), clean the affected port, and retry the installation. If it succeeds, the software in /usr/local was the culprit. Users are advised to install whatever software they need using MacPorts and remove it from /usr/local.

[afb@…]

This goes for /usr too, but presumably the vendor is the only one that install stuff there.