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/.clang-ignore b/.clang-ignore
new file mode 100644
index 0000000..36f17f2
--- /dev/null
+++ b/.clang-ignore
@@ -0,0 +1 @@
+./subprojects