I am using Ubuntu 12.04 / Eclipse 4.2.1 / CDT 8.1.1 and I used to have the same problem for quite some time: importing a C++ project from SVN would cause these annoying "Unresolved inclusion" errors and I would instead have to create a new project and copy the files in there as a work-around (still partial, since SVN functionality would not be there!).
At last, I have just found a simple, satisfactory solution:
Go to Project -> Properties -> C/C++ General -> Preprocessor Include Paths, Macros etc. -> Providers
and check Enable language settings providers for this project
.
Restart Eclipse.
Hopefully that already does the trick.