[CMake] Using generator expression with add_library

[David Aldrich]

I want to build a shared library for Linux and a static library for
Windows. So I have tried:

 set (_star_lib_name "StdStars")

 add_library(${_star_lib_name}
             $<$<CXX_COMPILER_ID:GNU>:SHARED>
             $<$<CXX_COMPILER_ID:MSVC>:STATIC>
             ""
 )

but that gives me error:

CMake Error at C:/SVNProj/zodiac/branches/TRY_TML_CMake_3Oct2019/StarLibs/StdStars/CMakeLists.txt:7
(add_library):
Cannot find source file:

    STATIC

  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .m .M .mm .h .hh .h++ .hm
  .hpp .hxx .in .txx

What is the correct way of doing this please?
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