Add option to gen_snapshot for creating a Makefile describing a snapshot's dependencies.

runtime/bin/file_macos.cc

Index: runtime/bin/file_macos.cc
diff --git a/runtime/bin/file_macos.cc b/runtime/bin/file_macos.cc
index 676f8c309ef80b804c2f07e3468775f121663015..7309da055fa272c3157f2bc90e7d0aa929ca51e1 100644
--- a/runtime/bin/file_macos.cc
+++ b/runtime/bin/file_macos.cc
@@ -122,6 +122,26 @@ int64_t File::Write(const void* buffer, int64_t num_bytes) {
 }
 
 
+bool File::VPrint(const char* format, va_list args) {
+  // Measure.
+  va_list measure_args;
+  va_copy(measure_args, args);
+  intptr_t len = vsnprintf(NULL, 0, format, measure_args);
+  va_end(measure_args);
+
+  char* buffer = reinterpret_cast<char*>(malloc(len + 1));
+
+  // Print.
+  va_list print_args;
+  va_copy(print_args, args);
+  vsnprintf(buffer, len + 1, format, print_args);
+  va_end(print_args);
+
+  bool result = WriteFully(buffer, len);
+  free(buffer);
+  return result;
+}
+
 int64_t File::Position() {
   ASSERT(handle_->fd() >= 0);
   return lseek(handle_->fd(), 0, SEEK_CUR);