Remove all #define _GNU_SOURCE in source files
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.
diff --git a/src/builtin.c b/src/builtin.c
index fb7513c..d486938 100644
--- a/src/builtin.c
+++ b/src/builtin.c
@@ -1,6 +1,3 @@
-#ifndef _GNU_SOURCE
-# define _GNU_SOURCE
-#endif
#ifndef __sun__
# define _XOPEN_SOURCE
# define _XOPEN_SOURCE_EXTENDED 1
diff --git a/src/compile.c b/src/compile.c
index 5645fa5..4af74ff 100644
--- a/src/compile.c
+++ b/src/compile.c
@@ -1,6 +1,3 @@
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE // for strdup
-#endif
#include <assert.h>
#include <math.h>
#include <string.h>
diff --git a/src/inject_errors.c b/src/inject_errors.c
index b61a271..a9fe9a8 100644
--- a/src/inject_errors.c
+++ b/src/inject_errors.c
@@ -1,5 +1,3 @@
-
-#define _GNU_SOURCE /* for RTLD_NEXT */
#include <assert.h>
#include <dlfcn.h>
#include <errno.h>
diff --git a/src/util.c b/src/util.c
index 250bdf7..a5c2b61 100644
--- a/src/util.c
+++ b/src/util.c
@@ -1,8 +1,3 @@
-
-#ifdef HAVE_MEMMEM
-#define _GNU_SOURCE
-#endif
-
#include <sys/types.h>
#include <sys/stat.h>
#include <assert.h>