Turn on a implicit-function-declaration warning by default.

A non declared function leads to a seriuos problems. And while
gcc don't turn this warning on lets tcc do it. This warning
can be turned off by -Wno-implicit-function-declaration option.
And autor must explicitly do this if program must be compiled
with this warning off.

win32/examples/fib.c

@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include <stdlib.h>	// atoi()
 
 int fib(n)
 {