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.

libtcc.c

@@ -1045,6 +1045,7 @@ LIBTCCAPI TCCState *tcc_new(void)
                                       ".dynhashtab", SHF_PRIVATE);
     s->alacarte_link = 1;
     s->nocommon = 1;
+    s->warn_implicit_function_declaration = 1;
 
 #ifdef CHAR_IS_UNSIGNED
     s->char_is_unsigned = 1;