Use a "fake" CONDITION_VARIABLE for mingw

It seems like the windows.h header file included through mingw
doesn't define CONDITION_VARIABLE causing the build to break.
Since the variable is only used to declare space, we can just make
a storage that's big enough for now.

Change-Id: Ica2433f6807576c8d469bf47c626c765fbe8c7a0
Reviewed-on: http://review.couchbase.org/29499
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>