Add warning if mingw gcc compiler is not installed

Author: linknum23

Makefile

@@ -1,5 +1,5 @@
 mercury2_prog: mercury2_prog.c
 	gcc -O3 -o bin/mercury2_prog mercury2_prog.c d2xx_linux/libftd2xx.a -pthread -ldl -Id2xx_linux
-	x86_64-w64-mingw32-gcc-win32 -O3 -o bin/mercury2_prog.exe mercury2_prog.c d2xx_win/ftd2xx.lib -Id2xx_win
+	test -x $(command -v x86_64-w64-mingw32-gcc-win32) && x86_64-w64-mingw32-gcc-win32 -O3 -o bin/mercury2_prog.exe mercury2_prog.c d2xx_win/ftd2xx.lib -Id2xx_win || echo "WARNING: missing x86_64-w64-mingw32-gcc-win32 for compiling windows executable"
 clean: 
 	rm -f bin/mercury2_prog