Yep,
I am definitively out of luck with FPCVERSION or FPCVER. Reading docs or googling I did not found *how* Lazarus resolves this macro, where it looks this information. 0.9.26 release notes says
FPC source directory can now use IDE macros, like the new FPCVer, so that switching the compiler automatically switches the source directory
Also, I can't install fpc 2.4 in the same base dir as 2.2 because of freepascal installer. To get fpc 2.4 installed in
/usr/lib/fpc/2.4.0/ next to 2.2.4, we have to set base install directory to
usr/ so that directories
bin/ man/ lib/ and
share/ go to the proper location, and doing that will erase all 2.2.4 binaries in
/usr/bin/ 
So in summary I can't use the first solution, nor the second one. So I started to use this workaround: instead of running svn lazarus with
startlazarus --pcp=~/.lazarussvn --scp=~/.lazarussvn
I use a shell script that :
- copy my fpc2.4.0.cfg file to ~/.fpc.cfg
- start svn lazarus as before
- delete ~/.fpc.cfg when I exit Lazarus
This way it use the right file and I can also run regular lazarus normally. I just can't run BOTH simultaneously.