Fix: Use more selectivity when building SSE specific code

See: https://github.com/OpenTTD/OpenTTD/issues/9935

(cherry picked from commit c3f55d53405af04bfe928d9e1a815e06c187ca50)

src/blitter/32bpp_sse2.hpp

@@ -16,6 +16,10 @@
 #define SSE_VERSION 2
 #endif
 
+#ifndef SSE_TARGET
+#define SSE_TARGET "sse2"
+#endif
+
 #ifndef FULL_ANIMATION
 #define FULL_ANIMATION 0
 #endif