cgo: full supports cgo preambles and auto compile c files

_demo/cgofull/foo.go

@@ -0,0 +1,16 @@
+package main
+
+/*
+#cgo CFLAGS: -DFOO
+#include <stdio.h>
+#include "foo.h"
+static void foo(Foo* f) {
+	printf("foo in bar: %d\n", f->a);
+}
+*/
+import "C"
+
+func Foo(f *C.Foo) {
+	C.print_foo(f)
+	C.foo(f)
+}