include cleanups: remove unused headers and add feature test macros

src/stdlib/ecvt.c

@@ -1,3 +1,4 @@
+#define _GNU_SOURCE
 #include <stdlib.h>
 #include <stdio.h>
 

src/stdlib/gcvt.c

@@ -1,3 +1,4 @@
+#define _GNU_SOURCE
 #include <stdlib.h>
 #include <stdio.h>
 

src/stdlib/wcstod.c

@@ -1,6 +1,7 @@
 #include "shgetc.h"
 #include "floatscan.h"
 #include "stdio_impl.h"
+#include <wchar.h>
 #include <wctype.h>
 
 /* This read function heavily cheats. It knows: