[C++] allow to explicitly avoid propagating CMake flags when using AddExternalProject
[C++] allow to explicitly avoid propagating CMake flags when using AddExternalProject

when adding an external dependency via AddExternalProject,
some compiler related CMake flags (CMAKE_C_COMPILER, CMAKE_CXX_COMPILER and
CMAKE_CXX_FLAGS), which have been set for the joynr CMake project,
are by default applied to that dependency as well.

if this behavior is not wanted (e.g. to avoid compiling the external dependency
with incompatible CXX_FLAGS), the newly added parameter "NO_PROPAGATE_CMAKE_ARGS"
can be given to AddExternalProject.

Change-Id: I0c41848ffb08ea932d69c23bc420e78e43815db0

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