meson: Prepare for subprojects

Google-Bug-Id: 179721407
Change-Id: I517dcb47d7c73fa4f3c7fcc25146b77de50d69b8
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/meson_options.txt b/meson_options.txt
new file mode 100644
index 0000000..0fc2767
--- /dev/null
+++ b/meson_options.txt
@@ -0,0 +1 @@
+option('tests', type: 'feature', description: 'Build tests')