Offscreen rendering done (faked) in the most compatible way :
render to backbuffer, and glTexCopySubImage to your texture. Easy, reasonably fast, and very well supported. But you need an actual visible windows, obscured parts are undefined, so this is not 'true offscreen rendering.
pbuffers : complex to program, but works on windows and apparently on linux too, check glx stuff (wglext.h is indeed windows only). You have to code both path differently though.
You may check my website and my tries on render-to-texture stuff, both with windows’ pbuffers and glCopyTexSubImage : http://www.chez.com/dedebuffer/