Make ex1.c and ex4.c be executable on any systems

Use /usr/bin/env to make ex1.c and ex4.c on any systems, wherever is
installed tcc.

examples/ex1.c

@@ -1,4 +1,4 @@
-#! /usr/local/bin/tcc -run
+#!/usr/bin/env tcc -run
 #include <tcclib.h>
 
 int main()

examples/ex4.c

@@ -1,4 +1,4 @@
-#!./tcc -run -L/usr/X11R6/lib -lX11
+#!/usr/bin/env tcc -run -L/usr/X11R6/lib -lX11
 #include <stdlib.h>
 #include <stdio.h>
 #include <X11/Xlib.h>