cmake 3.10 compatibility: pass absolute path to file(GENERATE) function

[mikaelarguedas]

This is needed to support cmake 3.10. We could define the policy based on the version of CMake but I preferred updating the codebase to have something compliant with all cmake versions without relying on undefined behavior.

More details at https://cmake.org/cmake/help/git-stage/policy/CMP0070.html

Example of CMake warning without this patch:

Policy CMP0070 is not set: Define file(GENERATE) behavior for relative
paths.  Run "cmake --help-policy CMP0070" for policy details.  Use the
cmake_policy command to set the policy and suppress this warning.

file(GENERATE) given relative OUTPUT path:

  test/test_services__rmw_connext_cpp_RelWithDebInfo.py

This is not defined behavior unless CMP0070 is set to NEW.  For
compatibility with older versions of CMake, the previous undefined behavior
will be used.

connects to ros2/rcl#195