configure: Fix MinGW detection

Fixes: bd8b313
       ("configure: Define _WIN32_WINNT when compiling with MinGW")
Ref: axel-download-accelerator#313
Signed-off-by: Ismael Luceno <ismael@iodev.co.uk>

configure.ac

@@ -163,7 +163,7 @@ hpux*)
        LDFLAGS="$LDFLAGS -L/usr/local/lib/hpux32"
     fi
 ;;
-*-mingw32)
+*mingw32)
     AC_DEFINE([_WIN32_WINNT], [0x0600],
               [Request NT 6 API to expose AI_ADDRCONFIG in WinSock2])
 ;;