Do not override CMake variables from target
Do not override CMake variables from target

In case of externally set variables for CMAKE_CXX_EXECUTABLE etc., they
are overridden again if the capnp_tool target has been found. This
change prevents this by checking if the variables are already set and
sets them from target only if they are empty.

Fixes #950.

Acked-by: Sven Fischer <dd860110a62b19214c4ee03aec0abffecb4e86b8@leiderfischer.de>

mologie/capnproto,mologie/capnproto,mologie/capnproto