meson: convert gtest to an internal dependency

meson: convert gtest to an internal dependency

In truth gtest is an external dependency that upstream expects you to
"vendor" into your own tree. As such, it makes sense to treat it more
like a dependency than an internal library, and collect it's
requirements together in a dependency object.

v2: - include with -isystem instead of setting compiler args (Eric)

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>