respect read-only source space

[fmessmer]

by executing the custom_targets in the build space rather than source space

read-only source space is enforced by e.g. https://github.com/ros-industrial/industrial_ci

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[fmessmer]

CLA done

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[jubeira]

Thanks for the submission @fmessmer; I'll give it a shot.

Could you explain a bit more the problem you addressed? Did the CI fail because the source space was not read-only?

[fmessmer]

CI failed because source space is read-only
and without this PR files were generated in the source space such as gradle cache etc.
with this PR all those files are generated in build space

[jubeira]

@fmessmer I see your point; the only thing that makes me a bit unsure about this is that building projects with Gradle wrapper (./gradlew build) or with Android Studio will output the artifact results to a different place than when building with catkin with these changes. In that sense, IMHO it should be optional to preserve the source space when building with catkin, not necessarily the default.

On the other hand, I talked with @ernestmc and he tends to agree with using the build space to dump the results as you propose. The motivation is that it's OK for catkin to behave the same for anything it's building, so there should be no difference between building a Gradle project or a python / C++ package.

I'll leave this open just a bit more; if there are no comments against it I will merge it after some tests.

[fmessmer]

closing in favor of #39 (same feature from my own fork - because I already merged #38 into mojin-robotics/rosjava_build_tools)