How can I set common cmake flags for all my packages?

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Two other 'alternatives':

1. make use of catkin_tools' profiles to store CMake and / or make flags (this basically adds some convenience to @dornhege's suggestion)
2. create a catkin package that exports your shared build flags in a CFG_EXTRAS file (see catkin - Developers Guide - Extracted CMake API reference - catkin_package() and this answer for some more information). This approach would require all of your other packages that need to use those flags to build_depend on your 'shared-flags' package and add them to their CMAKE_CXX_FLAGS variable though (but most of that can be automated).