Use the "-shared" option to link shared libraries. The compiler does not support "-Wl," or "-rpath" but does know how to pass "-soname" through to the linker.
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+SET(CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS "-shared")
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+SET(CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG "")
+SET(CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG_SEP "")
+SET(CMAKE_SHARED_LIBRARY_RPATH_LINK_C_FLAG "")
+SET(CMAKE_SHARED_LIBRARY_SONAME_C_FLAG "-soname ")