That's difficult. But you can get a very good approximation by changing to a package directory and running the following command:
make -dv show-var VARNAME=VARNAME \ | sed -n 's,^Global:\([^ ]*\) =.*,\1,p' \ | sed 's,\..*,.*,' \ | sort -u \ | grep ^\[A-Z\] \ | less
Another possibility is to run bmake show-all. This will list many (but not all) variables.
If you need more information about a specific variable, run bmake help topic=VARNAME or have a look at pkglint's variable definition file
If you want your changes to override everything else, then look for "ac_config_files=" and put it somewhere before that line.
In the file doc/CHANGES-*.
There is practically no difference. All the standard options are supported on all platforms. See also.