If you compile a kernel with options ACPI_DEBUG, you get

../../../../dev/acpi/acpica/Subsystem/acenv.h:270: warning: `DEBUGGER_THREADING' redefined
../../../../dev/acpi/acpica/Subsystem/acnetbsd.h:213: warning: this is the location of the previous definition

which -Werror isn't happy about. acnetbsd.h protects DEBUGGER_THREADING, but
acenv.h doesn't, so somehow the order of the includes should be reversed...

Cheers,

Patrick