A gcc preprocessor option -dD added

With this option on a defines are included into the output
(inside comments). This will allow to debug a problems like:

    In file included from math.c:8:
    In file included from /usr/include/math.h:43:
    /usr/include/bits/nan.h:52: warning: NAN redefined

tcc.c

@@ -47,6 +47,7 @@ static void help(void)
            "  -Usym       undefine 'sym'\n"
            "  -P          do not output a #line directive\n"
            "  -P1         use a #line directive in output instead of the gcc style\n"
+           "  -dD         put a define directive in the output (inside a comment)\n"
            "Linker options:\n"
            "  -Ldir       add library path 'dir'\n"
            "  -llib       link with dynamic or static library 'lib'\n"
@@ -307,6 +308,8 @@ int main(int argc, char **argv)
         start_time = getclock_us();
 
     tcc_set_output_type(s, s->output_type);
+    if (s->output_type == TCC_OUTPUT_PREPROCESS)
+	print_defines();
 
     /* compile or add each files or library */
     for(i = ret = 0; i < s->nb_files && ret == 0; i++) {