tools/power turbostat: Use $(CURDIR) instead of $(PWD) and add support for O= option in Makefile

Since commit ee0778a3
("tools/power: turbostat: make Makefile a bit more capable")
turbostat's Makefile is using

  [...]
  BUILD_OUTPUT    := $(PWD)
  [...]

which obviously causes trouble when building "turbostat" with

  make -C /usr/src/linux/tools/power/x86/turbostat ARCH=x86 turbostat

because GNU make does not update nor guarantee that $PWD is set.

This patch changes the Makefile to use $CURDIR instead, which GNU make
guarantees to set and update (i.e. when using "make -C ...") and also
adds support for the O= option (see "make help" in your root of your
kernel source tree for more details).

Link: https://bugs.gentoo.org/show_bug.cgi?id=533918
Fixes: ee0778a3 ("tools/power: turbostat: make Makefile a bit more capable")
Signed-off-by: Thomas D. <whissi@whissi.de>
Cc: Mark Asselstine <mark.asselstine@windriver.com>
Signed-off-by: Len Brown <len.brown@intel.com>

tools/power/x86/turbostat/Makefile

 CC		= $(CROSS_COMPILE)gcc
-BUILD_OUTPUT	:= $(PWD)
+BUILD_OUTPUT	:= $(CURDIR)
 PREFIX		:= /usr
 DESTDIR		:=
 
+ifeq ("$(origin O)", "command line")
+	BUILD_OUTPUT := $(O)
+endif
+
 turbostat : turbostat.c
 CFLAGS +=	-Wall
 CFLAGS +=	-DMSRHEADER='"../../../../arch/x86/include/uapi/asm/msr-index.h"'