Fix compiler warnings (Linux)

We suppress some 3rd-party code warnings, but it was done using
Windows compiler syntax. We fix it to do the same for gcc compiler
too.

src/image.cpp

@@ -27,13 +27,24 @@
 #include "image.h"
 #include "point.h"
 
-#pragma warning(push, 0)
+#if defined(_MSC_VER)
+  #pragma warning(push, 0)
+#else
+  #pragma GCC diagnostic push
+  #pragma GCC diagnostic ignored "-Wunused-parameter"
+#endif
+
 #define STB_IMAGE_IMPLEMENTATION
 #include "stb_image.h"
 
 #define STB_IMAGE_WRITE_IMPLEMENTATION
 #include "stb_image_write.h"
-#pragma warning (pop)
+
+#if defined(_MSC_VER)
+  #pragma warning (pop)
+#else
+  #pragma GCC diagnostic pop
+#endif
 
 namespace nkar
 {