Add a BUILD for gflags.

Change-Id: Id43e22c67645c301989cccab8b9f8043d8030355
diff --git a/third_party/empty_config_h/README b/third_party/empty_config_h/README
new file mode 100644
index 0000000..6db74f2
--- /dev/null
+++ b/third_party/empty_config_h/README
@@ -0,0 +1,4 @@
+This is a folder with an empty config.h file for external projects to #include.
+We #define everything relevant to each individual project using -D flags for
+each one, but files still want to #include "config.h". To allow that, add
+'-Ithird_party/empty_config_h' to copts.