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/meson.build b/meson.build
index 7edf7de..e089391 100644
--- a/meson.build
+++ b/meson.build
@@ -7,6 +7,7 @@
       'examples=' + (meson.is_subproject() ? 'disabled' : 'auto'),
     ],
     version: '1.0.0',
+    meson_version: '>=0.53.2',
 )
 
 libsystemd_pkg = dependency('libsystemd')