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/[email protected]
Signed-off-by: Edward Liaw <[email protected]>
Suggested-by: John Hubbard <[email protected]>
Acked-by: Shuah Khan <[email protected]>
Reviewed-by: Muhammad Usama Anjum <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: André Almeida <[email protected]>
Cc: Darren Hart <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Fenghua Yu <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Jarkko Sakkinen <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Kevin Tian <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Paul Walmsley <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Reinette Chatre <[email protected]>
Cc: Sean Christopherson <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>