If value not equal in cmake 2.8
===

**Short version:** I have build options that only work on one platform. The `autotools` file I'm converting form has a check of the form `if test "$platform_linux" != "yes"`. Can I do the same thing in my `CMakeLists.txt` (test if the value is NOT equal)?

**Slightly longer version:** I've got a test for various platforms following the advice found [here](http://www.openguru.com/2009/04/cmake-detecting-platformoperating.html):

<pre><code>
IF(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
    # Linux specific code
    SET(OperatingSystem "Linux")
ENDIF(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
</code></pre>

I'd like to do a test of the form `IF(${CMAKE_SYSTEM_NAME} NOT MATCHES "Linux")`. This doesn't appear to work, and the only documentation I can find is a mailing-list post from 2002, which suggests the `NOT` isn't valid for `cmake` prior to `1.2`. [[Link](http://www.cmake.org/pipermail/cmake/2002-January/002597.html)]. 

Is this still the case in later `cmake` versions, specifically `2.6` and/or `2.8`? 