meson: Explicitly check for CLI11 dependency

Change-Id: Ifaf6e5aba783285c16e8aae278e34dde6aa9d52c
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/.gitignore b/.gitignore
index a0155cb..327ea97 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 /build*/
+/subprojects/*/