Refine previous revision to allow acpi_wakecode.h to be safely built

from both the acpi module build directory and a kernel build directory.
The latter didn't work when one attempted to build a kernel which had
"device acpi" with the "make kernel-toolchain buildkernel" command
because a cross-compiler couldn't find anything in the standard system
include path (it's empty in the kernel-toolchain case).

Fix this by passing a better root path to kernel headers (src/sys)
which works for both cases, kernel and module (-I@ only worked for
module).

Also, while here, pass -nostdinc (and a different spelling for icc) --
it's a feature that the kernel source tree is self-contained, and this
change enforces this.

Reported by:	glebius

sys/i386/acpica/Makefile

@@ -9,7 +9,13 @@ DEPENDFILE=
 MAKESRCPATH= ${.CURDIR}
 CLEANFILES= acpi_wakecode.h acpi_wakecode.bin acpi_wakecode.o
 .endif
-CFLAGS+=	-I. -I@
+.if ${CC} == "icc"
+CFLAGS+=	-restrict
+NOSTDINC=	-X
+.else
+NOSTDINC=	-nostdinc
+.endif
+CFLAGS+=	${NOSTDINC} -I. -I${MAKESRCPATH}/../..
 
 all: acpi_wakecode.h