mail-client/thunderbird-9.0 is missing a few '#include <unistd.h>' statements, which causes the build to fail. The attached patch solved the problem for me. Reproducible: Always
Created attachment 298103 [details] emerge --info
Created attachment 298105 [details] bzipped build log
Created attachment 298107 [details, diff] Proposed patch
Patch belongs upstream so it will filter out before gcc-4.7 is released.