Split CFLAGS into smaller variables in NMakefile

Change-Id: I10d78ee89f90507dbb2a6e0c4443db3aa8524b1d
Reviewed-on: http://review.couchbase.org/28026
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>