Do not detect GNUstep as Cocoa
Do not detect GNUstep as Cocoa

GNUstep also installs Foundation/Foundation.h

https://bugzilla.gnome.org/show_bug.cgi?id=674172

Signed-off-by: William Hua <c824fe0afe16857dd6f587aa7c4044d2642d60fb@attente.ca>

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