--enable-plugin option overloaded

[Matthias Klose]

--enable-plugin is used by classpath (part of libjava) and now by GCC itself. 
disabling the build of the gcjwebplugin now disables plugin support in GCC as 
well. Please could the option for enabling GCC plugin support be renamed to 
something like --enable-plugins, --enable-gcc-plugin, --enable-gcc-plugins ? The 
only reason for not renaming the existing libjava option is that it was there 
first, and that it is part of an imported tree.

  Matthias