This is based on Lazarus trunk revision 64085 and FPC 3.2.0. That particular revision was chosen since later revisions are (currently?) not compilable with 3.0.4 and I anticipated that I might need a working IDE to debug this issue.
To progress past the initial setup dialogue with Lazarus trunk and FPC 3.2.0, (a recent) fpcmkcfg has to be on the path, and the "right answer" appears to be an fppkg.cfg file which has to be writeable (its modified timestamp is changed the first time the IDE starts). For example /usr/local/src_fpc/fpcbuild-3.2.0/fpcsrc/utils/fpcmkcfg/fppkg.cfg which the IDE can't locate by itself even if it has been told where the FPC sources are.
The IDE now appears to create a ~/.config/fppkg.cfg even if execution is qualified with a --pcp option, if this file is removed it is recreated the next time the IDE is run. These two .cfg files do not have the same content.
I think there are aspects of this which could do with being tidied up. In particular (a) starting Lazarus shouldn't update anything in the FPC source directory and (b) the working fppkg.cfg should be subject to the --pcp option.
MarkMLl