[WebKit2] Only preprocess sandbox profiles if we're building for the OS X SDK
https://bugs.webkit.org/show_bug.cgi?id=112330

Reviewed by David Kilzer.

We shouldn't preprocess sandbox profiles just because the computer
we're building on is a Mac. We should only do it if we're actually
building for the OS X SDK.

* DerivedSources.make: Check if $PLATFORM_NAME is macosx rather than if
$OS is MACOS.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145846 268f45cc-cd09-0410-ab3c-d52691b4dbfc