[CMake] INCLUDE() guards

[Nico Schlömer]

Hi all,

I have a project with multiple interdependent subpackages here, each
of which has its own export config file. The dependency hierarchy is
reflected in the `INCLUDE()` statements in the export files.

I would like to avoid that an export config file is included more than
once, so I'm guarding the files with
```
IF(NOT <name of the subpackage>_CONFIG_INCLUDED)
  SET(<name of the subpackage>_CONFIG_INCLUDED 1)
  [...]
  <setting of export variable etc>
  [...]
ENDIF()
```
This looks very much like the old C/C++ `#ifdef` `#include` header
guards, so I was wondering if this is an appropriate pattern for CMake
too. Or maybe there is another CMake command that already does exactly
what I want?

Cheers,
Nico