build: Allow for vendored googletest

This makes it easier to run the test suite outside of the CI
environment but still attempts to use the system version of googletest
first.

Change-Id: I32193f08243db2177b8200b0ff7e3e47ef01691f
Signed-off-by: William A. Kennington III <wak@google.com>
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/.gitignore b/.gitignore
index 24ddad5..5917395 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 # Meson build directories.
 /build*/
+/subprojects/*/
 
 # Created by https://www.gitignore.io/api/vim,python
 # Edit at https://www.gitignore.io/?templates=vim,python