Revert "selftests: Compile kselftest headers with -D_GNU_SOURCE"

This reverts commit daef47b8.

This framework change to add D_GNU_SOURCE to KHDR_INCLUDES
to Makefile, lib.mk, and kselftest_harness.h is causing build
failures and warnings.

Revert this change.
Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/Makefile

@@ -170,11 +170,11 @@ ifneq ($(KBUILD_OUTPUT),)
   # $(realpath ...) resolves symlinks
   abs_objtree := $(realpath $(abs_objtree))
   BUILD := $(abs_objtree)/kselftest
-  KHDR_INCLUDES := -D_GNU_SOURCE -isystem ${abs_objtree}/usr/include
+  KHDR_INCLUDES := -isystem ${abs_objtree}/usr/include
 else
   BUILD := $(CURDIR)
   abs_srctree := $(shell cd $(top_srcdir) && pwd)
-  KHDR_INCLUDES := -D_GNU_SOURCE -isystem ${abs_srctree}/usr/include
+  KHDR_INCLUDES := -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
-static_assert(0, "kselftest harness requires _GNU_SOURCE to be defined");
+#define _GNU_SOURCE
 #endif
 #include <asm/types.h>
 #include <ctype.h>

tools/testing/selftests/lib.mk

@@ -67,7 +67,7 @@ MAKEFLAGS += --no-print-directory
 endif
 
 ifeq ($(KHDR_INCLUDES),)
-KHDR_INCLUDES := -D_GNU_SOURCE -isystem $(top_srcdir)/usr/include
+KHDR_INCLUDES := -isystem $(top_srcdir)/usr/include
 endif
 
 # The following are built by lib.mk common compile rules.