KVM: selftests: Define _GNU_SOURCE for all selftests code

Define _GNU_SOURCE is the base CFLAGS instead of relying on selftests to
manually #define _GNU_SOURCE, which is repetitive and error prone.  E.g.
kselftest_harness.h requires _GNU_SOURCE for asprintf(), but if a selftest
includes kvm_test_harness.h after stdio.h, the include guards result in
the effective version of stdio.h consumed by kvm_test_harness.h not
defining asprintf():

  In file included from x86_64/fix_hypercall_test.c:12:
  In file included from include/kvm_test_harness.h:11:
 ../kselftest_harness.h:1169:2: error: call to undeclared function
  'asprintf'; ISO C99 and later do not support implicit function declarations
  [-Wimplicit-function-declaration]
   1169 |         asprintf(&test_name, "%s%s%s.%s", f->name,
        |         ^

When including the rseq selftest's "library" code, #undef _GNU_SOURCE so
that rseq.c controls whether or not it wants to build with _GNU_SOURCE.

Reported-by: Muhammad Usama Anjum <[email protected]>
Acked-by: Claudio Imbrenda <[email protected]>
Acked-by: Oliver Upton <[email protected]>
Acked-by: Anup Patel <[email protected]>
Reviewed-by: Muhammad Usama Anjum <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

0 files changed, 0 insertions, 0 deletions