INST: added target os for external libraries compilation
INST: added target os for external libraries compilation

Former-commit-id: a29656fb77dd47feea44f1be5137bcd6fa5a773a
Former-commit-id: 8a6218519df604c3cfdd8292a211ab3ae08f3a47
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox