Building examples with CMake does not respect features

[zayenz]

When building the examples using the CMake files, all examples are always built. This can be compared with the Makefile build, where examples are divided up into groups depending on their used features.

As a simple example of when this becomes a problem, if MPFR is not installed, then the sin, cos, and so on constraints for float variables are not available. However, the CMake build will still try to build the examples that require them (archimedean-spiraland golden-spiral) leading to a build error.