The global compiler settings are empty (I thought they are being override by project's settings anyway, don't they?), [...]
The global options are alwaqys added, but you can chose whether project options are added or prepended to the target options.
Compiler settings can be either in the "Compiler flags" or in the other "Other options" tab (for settings without checkbox).
And they can be made for the whole project, or for the target only (highlight the projects name or the targets name in the "Build options").
Please check all of these possibilities.
C::B does add build-flags on its own.