If you can build with components of your choosing, and install the OS of your choosing, then it is definitely part of the PC platform. The operating systems are like sub-platforms. Whether you are gaming on Windows or Linux, it is still PC gaming.
Mac is an iffy one because it is similar to PC, but it is a closed platform. However, you can install Windows on your Mac with Boot Camp, which would kind of make it part of the PC platform, but the hardware is generally locked down and you can not build a custom Mac. Fvck Apple anyway. LOL