CMake/Tests/OutOfSource/OutOfSourceSubdir/testlib.h

#ifdef _WIN32
#  ifdef testlib_EXPORTS
#    define CM_TEST_LIB_EXPORT  __declspec( dllexport )
#  else
#    define CM_TEST_LIB_EXPORT  __declspec( dllimport )
#  endif
#else
#  define CM_TEST_LIB_EXPORT 
#endif

CM_TEST_LIB_EXPORT float TestLib();