Minimising MacPorts Internal Disk Usage

[Bob M]

Hi all; I have an MB air with internal (smallish) SSD. I need some of
the programs provided by MacPorts at all times, but I'm happy to do
all builds while connected to a large-ish external HD. I just hit a
crisis when the /opt directories hit 27GB. So what I want to do is to
put as much as possible of the build environment onto the HD, and
substitute symbolic links on the SSD. I haven't tried to use it yet -
I'm assuming this is OK so long as nothing hard-links into these
directories.

The current status for the large directories in /opt is:
/opt/local/var/macports
    build                                    moved to HD
    distfiles                                    moved to HD
    software                                    not sure if this can be moved
/opt
    lib                                        I assume this can't be moved
    libexec                                    I assume this can't be moved
    share                                    I assume this can't be moved
(everything else is less than 250 MB, so not worth worrying about). So
far, I've saved about 12GB on the SSD, but that still leaves around
15GB (some of which may be Xcode-related rather than MacPorts, I'm not
sure where Xcode puts stuff). I'd like to go further if it's possible.
So I'd appreciate any suggestions, especially about
/opt/local/var/macports/software
    Thanks and Best WIshes
    Bob