Only use embedded GSL if there is no target "GSL"

[OlivierLDff]

When using stduuid with FetchContent/CPM.cmake/find_package, and also including GSL with such process, it create a conflict and it then become ambiguous which file is used where. Concrete use case:

CPMAddPackage(
  NAME GSL
  GIT_REPOSITORY "https://github.com/microsoft/GSL"
  GIT_TAG "v4.0.0"
)
CPMAddPackage(
  NAME stduuid
  GIT_REPOSITORY "https://github.com/mariusbancila/stduuid"
  GIT_TAG "master"
)

For now the workaround is to defined UUID_USING_CXX20_SPAN , which defeat its meaning.