building acpi module with ACPI_DEBUG option

If you build a kernel with the ACPI_DEBUG option, this option gets
passed to the module build in opt_acpi.h, but the kernel panics on boot
because the symbol AcpiDmDumpMethodInfo is undefined in the module.  The
problem is that the Makefile for the acpi module needs to know whether
ACPI_DEBUG is defined because it wants to append some more files to SRCS
in this case.  One of the extra source files contains the definition of
the undefined symbol.

.if ACPI_DEBUG
CFLAGS+=-DACPI_DEBUG
SRCS+=  ${DBSRC}
opt_ddb.h: Makefile
        echo "#define DDB 1" > ${.TARGET}
.else
opt_ddb.h: Makefile
        echo -n > ${.TARGET}
.endif