The issue was reported by Kuznetsov Eugene.

In my system /tmp/firebird is used by FB itself and has the owner
firebird:firebird.

If we compile fresh 2.5 not as root, we get

make[3]: Entering directory `/builds/firebird_250/firebird/gen'
make[3]: `../gen/firebird/bin/create_db' is up to date.
make[3]: Leaving directory `/builds/firebird_250/firebird/gen'
rm -f empty.fdb
../gen/firebird/bin/create_db empty.fdb
Can't access lock files' directory /tmp/firebird