Thanks to PMB and with a little fiddling around with the (new) SDK headers i managed to get METAMOD compiling in windows. At the same time this version should also work with Linux. It uses the new introduced int32 variable, instead of casting to __int32 (windows compatible only) only.
You can grab the 2 changed metamod source files here. Be sure you did pierre's fix for steam too to make sure it works!
