cyrus-sasl: remove trailing slash from plugin directory path

Specifying the plugin directory with a trailing '/' hit this failure in
the install rule:

  ... snip ...

  install-exec-local: $(INSTALLOSX)
        @if test "$(plugindir)" != "$(prefix)/lib/sasl2"; then \
          echo "********************************************************"; \
          echo "* WARNING:"; \
          echo "* Plugins are being installed into $(prefix)/lib/sasl2,"; \
          echo "* but the library will look for them in $(plugindir)."; \
          echo "* You need to make sure that the plugins will eventually"; \
          echo "* be in $(plugindir) -- the easiest way is to make a"; \
          echo "* symbolic link from $(plugindir) to $(prefix)/lib/sasl2,"; \
          echo "* but this may not be appropriate for your site, so this"; \
          echo "* installation procedure won't do it for you."; \
          echo "*"; \
          echo "* If you don't want to do this for some reason, you can"; \
          echo "* set the location where the library will look for plugins"; \
          echo "* by setting the environment variable SASL_PATH to the path"; \
          echo "* the library should use."; \
          echo "********************************************************"; \
        fi

Work around that.

Signed-off-by: Bian Naimeng <biannm@cn.fujitsu.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>