public: define XASH_MOBILE_PLATFORM by XASH_SAILFISH (defined externally)

public/build.h

@@ -138,7 +138,12 @@ Then you can use another oneliner to query all variables:
 	#endif
 #endif
 
-#if XASH_ANDROID || defined XASH_IOS || defined XASH_NSWITCH || defined XASH_PSVITA
+// XASH_SAILFISH is special: SailfishOS by itself is a normal GNU/Linux platform
+// It doesn't make sense to split it to separate platform
+// but we still need XASH_MOBILE_PLATFORM for the engine.
+// So this macro is defined entirely in build-system: see main wscript
+// HLSDK/PrimeXT/other SDKs users note: you may ignore this macro
+#if XASH_ANDROID || XASH_IOS || XASH_NSWITCH || XASH_PSVITA || XASH_SAILFISH
 	#define XASH_MOBILE_PLATFORM 1
 #endif