MB-11442: Specify macosx-version-min=10.7 when building ICU4C

Change-Id: Ib8a9d4a8f91d85bb1ed52acb680da79f7c551075
Reviewed-on: http://review.couchbase.org/38541
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>