pextlib1.0: Sort includes, declare getline(3)

Linux requires stdio.h for getline(3), and fails without it. glibc >=
2.10 defines getline(3) in _XOPEN_SOURCE >= 700. We ship a getline.h
compat implementation, but that doesn't get included unless HAVE_GETLINE
isn't defined, which isn't the case on Linux.

src/pextlib1.0/system.c

@@ -36,8 +36,8 @@
 #endif
 
 #ifndef __APPLE__
-/* required for fdopen(3)/seteuid(2), among others */
-#define _XOPEN_SOURCE 600
+/* required for fdopen(3)/seteuid(2)/getline(3), among others */
+#define _XOPEN_SOURCE 700
 #endif
 
 #include <tcl.h>
@@ -46,18 +46,19 @@
 #include <paths.h>
 #endif
 
-#include <sys/types.h>
-#include <sys/wait.h>
-#include <sys/resource.h>
 #include <assert.h>
+#include <errno.h>
 #include <fcntl.h>
+#include <limits.h>
+#include <signal.h>
 #include <stdbool.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/resource.h>
+#include <sys/types.h>
+#include <sys/wait.h>
 #include <unistd.h>
-#include <limits.h>
-#include <errno.h>
-#include <signal.h>
 
 #include "system.h"
 #include "Pextlib.h"