Fix up PLATFORM defs to match same-name ones in some SDK versions.

Some of the hl2sdk versions already have PLATFORM_WINDOWS, PLATFORM_LINUX, PLATFORM_POSIX, and PLATFORM_APPLE defined. We have guards around these on both sides.

However, the SDKs that have them define them as 1, rather than without a value. If SM's take precedence, we could be getting undesired behavior for any place in the SDKs that check these with #if rather than #ifdef.