How to install package in different directory prefix

["Thomas Mueller" <mueller6724%bellsouth.net@localhost>]

How, using pkgsrc or otherwise, would I setup to install a package in a 
different directory prefix, other than /usr, /usr/pkg, or /usr/local, and then 
how would I tell pkgsrc and gcc to recognize those installed libraries and 
includes if such an installed package becomes a dependency of something else I 
want to install?  I could have a prefix like /usr/local2 or /usr/testing and 
still have access to the base system and other installed packages.

I might want to install something in an experimental directory prefix to be 
able to access it but without burning bridges on an established installation.  
I could modify the path and library and include search paths for a special 
session but be able to subsequently return to normal setup.

I might want to do this with an alpha or beta prerelease of something like 
Firefox 4 or Seamonkey 2.1, for instance.

Tom