Fix compiling of 32 bit binary on 64-bit Windows

Two versions of mingw-w64 (targeting Win64 and Win32)
can be installed on Windows too.

No functional change

Resolves official-stockfish#532

src/Makefile

@@ -160,22 +160,18 @@ endif
 ifeq ($(COMP),mingw)
 	comp=mingw
 
-	ifeq ($(UNAME),Linux)
-		ifeq ($(bits),64)
-			ifeq ($(shell which x86_64-w64-mingw32-c++-posix),)
-				CXX=x86_64-w64-mingw32-c++
-			else
-				CXX=x86_64-w64-mingw32-c++-posix
-			endif
+	ifeq ($(bits),64)
+		ifeq ($(shell which x86_64-w64-mingw32-c++-posix),)
+			CXX=x86_64-w64-mingw32-c++
 		else
-			ifeq ($(shell which i686-w64-mingw32-c++-posix),)
-				CXX=i686-w64-mingw32-c++
-			else
-				CXX=i686-w64-mingw32-c++-posix
-			endif
+			CXX=x86_64-w64-mingw32-c++-posix
 		endif
 	else
-		CXX=g++
+		ifeq ($(shell which i686-w64-mingw32-c++-posix),)
+			CXX=i686-w64-mingw32-c++
+		else
+			CXX=i686-w64-mingw32-c++-posix
+		endif
 	endif
 
 	CXXFLAGS += -Wextra -Wshadow