Fix parsing of command-line arguments when already specified as a cmake list.

The built-in separate_arguments chokes when given a cmake list variable - it needs a space-separated list of arguments. However, this leads to an incompatibilty with our CI, in which the YAML configuration doesn't support forwarding space-separated variables along to build scripts (it either escapes the spaces, or separates spaces into different arguments). By allowing us to specify flags via semi-colon separated lists, these are already separated by cmake automatically. This also allows us to set these lists directly via the cmake command-line.