selftests: centralize -D_GNU_SOURCE= to CFLAGS in lib.mk

selftests: centralize -D_GNU_SOURCE= to CFLAGS in lib.mk

Centralize the _GNU_SOURCE definition to CFLAGS in lib.mk.  Remove
redundant defines from Makefiles that import lib.mk.  Convert any usage of
"#define _GNU_SOURCE 1" to "#define _GNU_SOURCE".

This uses the form "-D_GNU_SOURCE=", which is equivalent to
"#define _GNU_SOURCE".

Otherwise using "-D_GNU_SOURCE" is equivalent to "-D_GNU_SOURCE=1" and
"#define _GNU_SOURCE 1", which is less commonly seen in source code and
would require many changes in selftests to avoid redefinition warnings.

Link: https://lkml.kernel.org/r/20240625223454.1586259-2-edliaw@google.com
Signed-off-by: Edward Liaw <edliaw@google.com>
Suggested-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: André Almeida <andrealmeid@igalia.com>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Kees Cook <kees@kernel.org>
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Reinette Chatre <reinette.chatre@intel.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>