How to add additional compile flags

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$ROS_ROOT/core/rosbuild/rosconfig.cmake is where ros configuration starts. There are three places ros looks for rosconfig.cmake, and if you parse rosbuild's rosconfig.cmake, you'll find it essentially does the following:

• $ROS_ROOT/rosconfig.cmake - this is the user (global) configuration file.
• <pkg>/rosconfig.cmake - overrides the user's configuration file for a single package.

If neither of the above are found, or not all configuration variables are set:

• rosbuild/rosconfig.cmake finally sets some sane defaults.

If you're cross compiling and want to add cpu specific flags globally, you may want to use eric's suggestion and include them in $ROS_ROOT/rosconfig.cmake. This is neater as it is under your control and avoids hacking someone else's CMakeLists.txt files (which will disappear next time you install).

If you need it for a certain package only, then that would be the only way to go, though I'd suggest finding a way to upstream the changes so others can benefit as well.