Our SVN tree is here: svn co svn://gcc.gnu.org/svn/gcc/branches/plugin gcc
This is an old branch off the GCC 4.3 mainline that Eric Christopher helped create after the 2007 GCC Summit, but we can update it if necessary. It includes libtool and libltdl, which may be useful.
FWIW, the libtool/libltdl setup allows us to build and use plug-ins on all GNU/Linux platforms we've tested as well as Solaris and Mac OS X.
Sean
