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.

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>