selftests: Compile kselftest headers with -D_GNU_SOURCE

Add the -D_GNU_SOURCE flag to KHDR_INCLUDES so that it is defined in a
central location.

Commit 8092162335 ("selftests/harness: remove use of LINE_MAX")
introduced asprintf into kselftest_harness.h, which is a GNU extension
and needs _GNU_SOURCE to either be defined prior to including headers or
with the -D_GNU_SOURCE flag passed to the compiler.

Fixed up commit log:
Shuah Khan <skhan@linuxfoundation.org>

Fixes: 8092162335 ("selftests/harness: remove use of LINE_MAX")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202404301040.3bea5782-oliver.sang@intel.com
Signed-off-by: Edward Liaw <edliaw@google.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/Makefile

@@ -161,11 +161,11 @@ ifneq ($(KBUILD_OUTPUT),)
   # $(realpath ...) resolves symlinks
   abs_objtree := $(realpath $(abs_objtree))
   BUILD := $(abs_objtree)/kselftest
-  KHDR_INCLUDES := -isystem ${abs_objtree}/usr/include
+  KHDR_INCLUDES := -D_GNU_SOURCE -isystem ${abs_objtree}/usr/include
 else
   BUILD := $(CURDIR)
   abs_srctree := $(shell cd $(top_srcdir) && pwd)
-  KHDR_INCLUDES := -isystem ${abs_srctree}/usr/include
+  KHDR_INCLUDES := -D_GNU_SOURCE -isystem ${abs_srctree}/usr/include
   DEFAULT_INSTALL_HDR_PATH := 1
 endif
 

tools/testing/selftests/kselftest_harness.h

@@ -51,7 +51,7 @@
 #define __KSELFTEST_HARNESS_H
 
 #ifndef _GNU_SOURCE
-#define _GNU_SOURCE
+static_assert(0, "kselftest harness requires _GNU_SOURCE to be defined");
 #endif
 #include <asm/types.h>
 #include <ctype.h>

tools/testing/selftests/lib.mk

@@ -53,7 +53,7 @@ selfdir = $(realpath $(dir $(filter %/lib.mk,$(MAKEFILE_LIST))))
 top_srcdir = $(selfdir)/../../..
 
 ifeq ($(KHDR_INCLUDES),)
-KHDR_INCLUDES := -isystem $(top_srcdir)/usr/include
+KHDR_INCLUDES := -D_GNU_SOURCE -isystem $(top_srcdir)/usr/include
 endif
 
 # The following are built by lib.mk common compile rules.