configure: Add dll name recognition for cygwin.

aclocal.m4

@@ -42,7 +42,7 @@ AC_CACHE_VAL(ac_Lib,
 LIBS="-l$1 $5 $LIBS"
   AC_LINK_IFELSE([AC_LANG_CALL([], [$2])],
   [case "$LIBEXT" in
-    dll) ;;
+    dll) AS_VAR_SET(ac_Lib,[`$ac_cv_path_LDD conftest.exe | grep "$1" | sed -e "s/dll.*/dll/"';2,$d'`]) ;;
     dylib) AS_VAR_SET(ac_Lib,[`otool -L conftest$ac_exeext | grep "ac_lib_pattern\\.[[0-9A-Za-z.]]*dylib" | sed -e "s/^.*\/\(ac_lib_pattern\.[[0-9A-Za-z.]]*dylib\).*$/\1/"';2,$d'`]) ;;
     *) AS_VAR_SET(ac_Lib,[`$ac_cv_path_LDD conftest$ac_exeext | grep "ac_lib_pattern\\.$LIBEXT" | sed -e "s/^.*\(ac_lib_pattern\.$LIBEXT[[^	 ]]*\).*$/\1/"';2,$d'`]) ;;
   esac])