Fix detection of Windows in build script.

Thanks go to Noelle Leigh <noelle_leigh@fastmail.com> for the initial
report and much help during diagnosis!

build.sh

@@ -29,7 +29,7 @@ mkdir -p bin
 CC="${CC:-cc}"
 CFLAGS="${CFLAGS:--std=c89 -Wall -Wno-unknown-pragmas}"
 case "$(uname -s 2>/dev/null)" in
-MSYS_NT*) # MSYS2 on Windows
+MSYS_NT*|MINGW*) # MSYS2 on Windows
 	UXNEMU_LDFLAGS="-static $(sdl2-config --cflags --static-libs)"
 	;;
 Darwin) # macOS