the specific problem addressed here can (and IMHO should) be solved in
another way: remove the inclusion of the offending headers in gcc-common.h
as neither tm.h nor c-common.h are needed by existing plugins. for background,
We can't build without tm.h: http://pastebin.com/W0azfCr0
you'll need to repeat the removal of dependent headers. based on a quick
test here across gcc 4.5-6.2, if you remove rtl.h, tm_p.h, hard-reg-set.h
and emit-rtl.h in addition to tm.h, the plugins should build fine.