The SFML target export set includes a number of external targets
which are not owned by the project itself. This includes targets
like Freetype and OpenGL. By specifying a namespace for the export
set, a SFML:: namespace was prepended to all targets. This is not
a problem when using shared libraries but when building and using
static libraries caused a problem where CMake was attempting and
failing to find targets with names like SFML::Freetype or
SFML::OpenGL which did not exist.
Luckily CMake allows you put namespaces in the EXPORT_NAME target
property so now we can just add the SFML:: namespace in the macro
which creates SFML targets and remove the `NAMESPACE SFML::` line
which was adding namespaces to all targets.
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