This is not a problem, but a solution for those of you that have had the same problem as I have had. (This is all on Ubuntu Karmic)

The problem:
Whenever you attempt to compile an Objective-C file with clang that imports <objc/objc.h>, you get the error message in this post's title.

The fix:
For some strange reason, the objc include folder is located in a really strange place. So type
Code:
locate objc.h
. If this comes up with nothing, either try
Code:
sudo updatedb && locate objc.h
or install gobjc, then
Code:
sudo updatedb && locate objc.h
.

Mine looks like this:
Code:
jordy@data:~$ locate objc.h
/usr/lib/gcc/i486-linux-gnu/4.4/include/objc/objc.h
We want to link the include directory into /usr/local/include

So use this command:
Code:
sudo ln -s <your-objc-directory> /usr/local/include/objc
So, for example, mine looked like this:
Code:
jordy@data:~$ sudo ln -s /usr/lib/gcc/i486-linux-gnu/4.4/include/objc/ /usr/local/include/objc
Now after having done that, your compiles should go much smoother.