As of build 38285 BC Classic now has its own WOW_PROJECT_ID of 5 so addons that have conditional code for the different client flavors can simplify their checks.
Code:
WOW_PROJECT_MAINLINE = 1
WOW_PROJECT_CLASSIC = 2
WOW_PROJECT_BURNING_CRUSADE_CLASSIC = 5
Checking the global WOW_PROJECT_ID against one of the above is the simplest way to detect the client.