Removes the usage of __LINUX__ define

This is defined and set it a ton of different places.
It's checked in a whole lot more
Instead just use __linux__ like a real project should

plugins/GSnull/GS.cpp

@@ -46,7 +46,7 @@ const char* s_iniFilename = "GSnull.ini";
 GSVars gs;
 
 // Because I haven't bothered to get GSOpen2 working in Windows yet in GSNull.
-#ifdef __LINUX__
+#ifdef __linux__
 #define USE_GSOPEN2
 #endif