Ensure that -fno-strict-aliasing is _appended_ after -O2

-O2 includes -fstrict-aliasing, so we need to ensure that these flags
are explicitly added after each invocation. Maybe we should just use
unions instead *sigh*.

Change-Id: I9321609a671339dfe13502fdd071ad1de823bf9f
Reviewed-on: http://review.couchbase.org/43720
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>