commit 7ed1c1901fe52e6c5828deb155920b44b0adabb1 upstream.

Currently a number of Makefiles break when used with toolchains that
pass extra flags in CC and other cross-compile related variables (such
as --sysroot).

Thus we get this error when we use a toolchain that puts --sysroot in
the CC var:

~/src/linux/tools$ make iio
[snip]
iio_event_monitor.c:18:10: fatal error: unistd.h: No such file or directory
#include
^~~~~~~~~~

This occurs because we clobber several env vars related to
cross-compiling with lines like this:

CC = $(CROSS_COMPILE)gcc

Although this will point to a valid cross-compiler, we lose any extra
flags that might exist in the CC variable, which can break toolchains
that rely on them (for example, those that use --sysroot).

This easily shows up using a Yocto SDK:

$ . [snip]/sdk/environment-setup-cortexa8hf-neon-poky-linux-gnueabi

$ echo $CC
arm-poky-linux-gnueabi-gcc -march=armv7-a -mfpu=neon -mfloat-abi=hard
-mcpu=cortex-a8
--sysroot=[snip]/sdk/sysroots/cortexa8hf-neon-poky-linux-gnueabi

$ echo $CROSS_COMPILE
arm-poky-linux-gnueabi-

$ echo ${CROSS_COMPILE}gcc
krm-poky-linux-gnueabi-gcc

Although arm-poky-linux-gnueabi-gcc is a cross-compiler, we've lost the
--sysroot and other flags that enable us to find the right libraries to
link against, so we can't find unistd.h and other libraries and headers.
Normally with the --sysroot flag we would find unistd.h in the sdk
directory in the sysroot:

$ find [snip]/sdk/sysroots -path '*/usr/include/unistd.h'
[snip]/sdk/sysroots/cortexa8hf-neon-poky-linux-gnueabi/usr/include/unistd.h

The perf Makefile adds CC = $(CROSS_COMPILE)gcc if and only if CC is not
already set, and it compiles correctly with the above toolchain.

So, generalize the logic that perf uses in the common Makefile and
remove the manual CC = $(CROSS_COMPILE)gcc lines from each Makefile.

Note that this patch does not fix cross-compile for all the tools (some
have other bugs), but it does fix it for all except usb and acpi, which
still have other unrelated issues.

I tested both with and without the patch on native and cross-build and
there appear to be no regressions.

Link: http://lkml.kernel.org/r/20180107214028.23771-1-martin@martingkelly.com
Signed-off-by: Martin Kelly
Acked-by: Mark Brown
Cc: Tejun Heo
Cc: Li Zefan
Cc: Johannes Weiner
Cc: Linus Walleij
Cc: "K. Y. Srinivasan"
Cc: Haiyang Zhang
Cc: Stephen Hemminger
Cc: Jonathan Cameron
Cc: Pali Rohar
Cc: Richard Purdie
Cc: Jacek Anaszewski
Cc: Pavel Machek
Cc: Peter Zijlstra
Cc: Ingo Molnar
Cc: Arnaldo Carvalho de Melo
Cc: Robert Moore
Cc: Lv Zheng
Cc: "Rafael J. Wysocki"
Cc: Greg Kroah-Hartman
Cc: Valentina Manea
Cc: Shuah Khan
Cc: Mario Limonciello
Signed-off-by: Andrew Morton
Signed-off-by: Linus Torvalds
Cc: Ignat Korchagin
Signed-off-by: Greg Kroah-Hartman