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.

tests/tests2/64_macro_nesting.c

@@ -1,3 +1,5 @@
+#include <stdio.h>	// printf()
+
 #define CAT2(a,b) a##b
 #define CAT(a,b) CAT2(a,b)
 #define AB(x) CAT(x,y)