Allow prevent prescan to be re-entrant

[LeStarch]

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Has Unit Tests (y/n)
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Change Description

Prevent prescan fails when the library has already been defined. This is normally not an issue, but affects some edge-cases. This allows users to avoid a nasty if-tree when using prevent-prescan.

It fixes the problem shown here where normal target creation can be safe, but not so with prevent-prescan.

prevent_prescan(T1 T2)

if (NOT TARGET T1)
   add_library(T1)
endif()


if (NOT TARGET T2)
   add_library(T2)
endif()