do not define __unix in common.gypi

xlC defines both __unix and __unix__
(gcc on AIX did not define these in gcc versions prior to 4.7.0)
xcodebuild/clang does -not- define __unix or __unix__,
but bsl/bsls/bsls_platform.h is being modified to not require them
(Depends on https://github.com/bloomberg/bsl/issues/97)  (This patch also follows up on https://github.com/bloomberg/bsl/issues/74)