Cygwin build fixes

Fix mklib to deal with NOPREFIX and use --enable-auto-image-base for cygwin
Teach configure.ac some basic facts about cygwin

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
diff --git a/configure.ac b/configure.ac
index 48b8022..2a9ac1e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -222,6 +222,8 @@
     case "$host_os" in
     darwin* )
         LIB_EXTENSION='dylib' ;;
+    cygwin* )
+        LIB_EXTENSION='dll' ;;
     * )
         LIB_EXTENSION='so' ;;
     esac
@@ -1073,6 +1075,9 @@
     solaris*)
         APP_LIB_DEPS="-lX11 -lsocket -lnsl -lm"
         ;;
+    cygwin*)
+        APP_LIB_DEPS="-lX11"
+        ;;
     *)
         APP_LIB_DEPS="-lm"
         ;;