Add compatibility note: "AddOutputFilterByType" is still a core module in Apache 2.2

[fhemberger]

See issue #1256.

This adds at least a compatibility notice for Apache < 2.3.7.

There may be also two alternatives to improve this check further:
a) Check the API_VERSIONof the httpd module (docs):

# API Version for first Apache 2.3.7 dev release
<If "%{API_VERSION} -lt 20100609">
    # AddOutputFilterByType …
</If>

While this is documented, it didn't work on my default Apache on OS X Snow Leopard (v.2.2.2).

b) Check the version number with mod_version:

<IfModule mod_version.c>
    <IfVersion < 2.3.7>
        # AddOutputFilterByType …
    </IfVersion>
</IfModule>

but I'm not sure if mod_version is normally installed on production environments.

[alrra]

a) ... While this is documented, it didn't work on my default Apache on OS X Snow Leopard (v.2.2.2).

That is because the <If> directive was introduced starting with Apache v2.4.

b) Check the version number with mod_version:

mod_version doesn't bring any value over mod_filter, in both cases the user needs to enable 1 more module (plus, I think mod_filter is more useful then mod_version, which only has the IfVersion directive and you kinda have to duplicate the code - see #1012).

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@fhemberger merged, thank you! (hope you don't mind that I've modified the commit to make it a bit more clearer).