_WINDOWS_ is pre-defined macro definition for Windows OS Other libraries detect build on Windows by this macro Fix SLADE with wxWidgets 3.2 Index: FreeImage/Source/FreeImage.h =================================================================== --- FreeImage.orig/Source/FreeImage.h +++ FreeImage/Source/FreeImage.h @@ -129,7 +129,6 @@ FI_STRUCT (FIMULTIBITMAP) { void *data; #endif // __MINGW32__ #ifndef _WINDOWS_ -#define _WINDOWS_ #ifndef FALSE #define FALSE 0