I've had a look at /etc/fpc.cfg and there is mention to some defines:
Then in the fcl-web examples there is mention to:
The only one I've found is on the apr.pas on both these files:
So how does the compiler decide what version of APR to compile?
If you enable the right one of those conditionals/directives afaik, the rest of the fpc.cfg should put the corresponding httpd dir first in the compilers search path, so that that version gets taken.
So you should only define one of them.