There are two types of #include statements: #include <foo.h> and #include "foo.h".
Say we have the file xyz.h in /usr/include/mylib/ that contains the following:
The preprocessor will search for the file header1.h in all the paths given as -I arguments and then replace the line with the contents of that file.
For line 2 the preprocessor tries to use the file /usr/include/mylib/header2.h first and if it does not exist search for the file like it did for header1.h. The important part to note here is that the preprocessor does not look in the directory of the source file that includes xyz.h but in the directory where xyz.h resides.
Now, which include statement is the one to use? After all you can specify every directory you want using -I and thus could use #include <...> everywhere.
Rationale: The header files of external libraries are obviously not in the same directory as your source files. So you need to use angle brackets.
Headers of your own application have a defined relative location to the source files of your application. Using KDE4's cmake macros your source directory is the first include switch to the compiler and therefore there's no difference in using angle brackets or double quotes. If you work with a different buildsystem that does not include the current source directory or disable CMAKE_INCLUDE_CURRENT_DIR then all includes (inside your application) using angle brackets will break.Ideally the buildsystem would not need to specify -I