disable sane's libtool soname hack on mingw

Original logic was confusing the DLL creation process.
Each backend was creating a DLL of name libsane-1.dll;
overwriting each other each time.  Since libsane is built
last, you could never get access to other DLL's like
you can on unix platforms.

Windows doesn't use soname concept anyways.  You can copy any
libsane-*-1.dll to libsane-1.dll into same directory
as scanimage.exe and it will work as expected for that
one backend.

ltmain.sh

@@ -8111,6 +8111,8 @@ EOF
 	  dlname="$soname"
 	fi
 	case $host in
+	  *mingw*)
+	    ;;
 	  *aix*)
 	    ;;
 	  *)