Good news!
Sure this will be helpful for other users.
Actually, DS exports environment variables with the exact names mentioned in the preferences pages (e.g. RVCT40LIB, etc…)
Note that if ARMLIB variable is required by your compiler, maybe it is closer to ADS configuration system (cf. ADS preference page in DS, where you can configure ARMLIB & other variables…)
Last option, instead of modifying your OS environment, you can configure DS/CDT to make it export environment variables at build time: cf Window > Preferences > C/C++ > Build > Environment. You can even overwrite these global settings at project level in each project properties.