[CMake] Include paths not being searched?

[David Doria]

The reason I was looking to check the contents of INCLUDE_DIRECTORIES
was that when I build my project I am getting errors like this:

/usr/include/fstream:40:19: error: istream: No such file or directory

The output of this:
get_property(inc_dirs DIRECTORY PROPERTY INCLUDE_DIRECTORIES)
message("inc_dirs = ${inc_dirs}")

indicates that there are several directories on INCLUDE_DIRECTORIES
that contain istream

For example, this one:
/usr/include/c++/4.4.1

contains the file:
[doriad at davedesktop 4.4.1]$ cd /usr/include/c++/4.4.1
[doriad at davedesktop 4.4.1]$ ls is*
istream

Does anyone know what else could cause this type of file (it's not
only istream, but also ostream, cstdio, etc) not to be found?

Thanks,

David