[C++,Build] Make platform provided gtest and gmock possible
[C++,Build] Make platform provided gtest and gmock possible

For some build systems like Yocto it it necessary to be able to
provide third party dependencies via the sysroot. This enables taking
gtest and gmock from the platform if USE_PLATFORM_GTEST_GMOCK is enabled.

Change-Id: I64f574cefbacc49e5cd555e9fbfaca3add74d367

bmwcarit/joynr,clive-jevons/joynr,clive-jevons/joynr,clive-jevons/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr