Not an expert in this, but my recollection of C is that it has two kinds of includes, the <> and the "". The <> are system ones that are fairly hard to override and not part of make dependencies, while the "" ones are those of your app and sensitive to the include path. So it all depends how the GTK stuff is included.
I did compile a 2.8 version not so long ago and it was quite easy. Why do you need to import foreign GTK includes? Either you have GTK2 installed on your system (in which case adequate includes would come with some "-dev" package) or you don't, in which case Gimp won't run even if you manage to spoon-feed includes during compilation.
Finally, I wonder why the 2.6 fetish. Things have improved a lot since 2012. Some users are irked by some design choices (such as the save/export controversy), but IMHO if you are down to compiling something to circumvent this, you can as well compile a recent version in which you make some tweaks (there are also scripts & plugins).
I did compile a 2.8 version not so long ago and it was quite easy. Why do you need to import foreign GTK includes? Either you have GTK2 installed on your system (in which case adequate includes would come with some "-dev" package) or you don't, in which case Gimp won't run even if you manage to spoon-feed includes during compilation.
Finally, I wonder why the 2.6 fetish. Things have improved a lot since 2012. Some users are irked by some design choices (such as the save/export controversy), but IMHO if you are down to compiling something to circumvent this, you can as well compile a recent version in which you make some tweaks (there are also scripts & plugins).