Add a BUILD for gflags.

Change-Id: Id43e22c67645c301989cccab8b9f8043d8030355
diff --git a/build_tests/BUILD b/build_tests/BUILD
new file mode 100644
index 0000000..e11937b
--- /dev/null
+++ b/build_tests/BUILD
@@ -0,0 +1,10 @@
+cc_test(
+  name = 'gflags_build_test',
+  srcs = [
+    'gflags.cc',
+  ],
+  deps = [
+    '//third_party/gflags',
+  ],
+  size = 'small',
+)
diff --git a/build_tests/gflags.cc b/build_tests/gflags.cc
new file mode 100644
index 0000000..150e330
--- /dev/null
+++ b/build_tests/gflags.cc
@@ -0,0 +1,7 @@
+#include "third_party/gflags/include/gflags/gflags.h"
+
+DEFINE_int32(test_flag, 0, "Test command-line flag");
+
+int main() {
+  return FLAGS_test_flag;
+}