cgo: support function-like macros

This is needed for code like this:

    #define __WASI_ERRNO_INVAL (UINT16_C(28))
    #define EINVAL __WASI_ERRNO_INVAL

cgo/testdata/const.go

@@ -3,13 +3,26 @@ package main
 /*
 #define foo 3
 #define bar foo
+
 #define unreferenced 4
 #define referenced unreferenced
+
+#define fnlike() 5
+#define fnlike_val fnlike()
+#define square(n) (n*n)
+#define square_val square(20)
+#define add(a, b) (a + b)
+#define add_val add(3, 5)
 */
 import "C"
 
 const (
 	Foo = C.foo
 	Bar = C.bar
+
 	Baz = C.referenced
+
+	fnlike = C.fnlike_val
+	square = C.square_val
+	add    = C.add_val
 )