Support using existing library builds of gmock and gtest

[Conan-Kudo]

gtest and gmock (part of Google Test) have not required being built from source to leverage them for years.

The Fedora package has offered libraries for a while now, and the Debian package does as well in stretch-backports and higher (buster and sid, too).

Please support using pre-built libraries for Google Mock and Google Test.

[QuLogic]

The dependency should check for pkgconfig files already.

[Conan-Kudo]

The documentation indicates otherwise: http://mesonbuild.com/Dependencies.html#gtest-and-gmock

[wak-google]

It does do the correct thing with using system sources assuming you aren't doing something like including them as a subproject. We have been using an external googletest / mock for a while in our builds.