This is unnecessary since we are using AC_USE_SYSTEM_EXTENSIONS in
configure.ac, which makes the compiler always run with -D_GNU_SOURCE=1
when building for GNU/Linux.
Also, the #define _GNU_SOURCE in src/util.c, and in
src/inject_errors.c (--enable-inject-errors), were not guarded by a
#ifndef _GNU_SOURCE so they were just triggering redefinition warnings.
Emanuele Torre