Revise CMake build instructions for v3.16

* Add optional Yase_USE_SYSTEM_GTEST
* Use CMAKE_INSTALL_PREFIX

Signed-off-by: Martin Stump <martin.stump@mercedes-benz.com>

README.md

@@ -39,7 +39,7 @@ Once the container started, it is possible to build the available packages and r
 
 Via CMake:
 ``` shell
-cmake -B build [--install-prefix=<path/to/prefix>]
+cmake -B build [-DCMAKE_INSTALL_PREFIX=<path/to/prefix>] [-DYase_USE_SYSTEM_GTEST=ON]
 cmake --build build [--target test]
 cmake --install build
 ```