Drop cmake anti-pattern
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This removes an anti-pattern from CMakeLists.txt (I'm very sorry for introducing it). The most annoying part about it is that it makes it difficult for a parent project (e.g. ICON) to support multi-configuration backends properly. Basically, the only right place to set CMAKE_BUILD_TYPE is on the command line when calling cmake (and generating for a single-configuration backend). Also, see here.