FindSodium: Do not create target unconditionally (#430)

Summary:
This fixes  https://github.com/facebook/fbthrift/issues/429

Pull Request resolved: https://github.com/facebook/fbthrift/pull/430

Reviewed By: iahs

Differential Revision: D28832985

Pulled By: vitaut

fbshipit-source-id: 0719f27207d11bb7970cc43c621640c425d8f55d

build/fbcode_builder/CMake/FindSodium.cmake

@@ -254,7 +254,10 @@ if(sodium_USE_STATIC_LIBS)
 else()
     set(_LIB_TYPE SHARED)
 endif()
-add_library(sodium ${_LIB_TYPE} IMPORTED)
+
+if(NOT TARGET sodium)
+    add_library(sodium ${_LIB_TYPE} IMPORTED)
+endif()
 
 set_target_properties(sodium PROPERTIES
     INTERFACE_INCLUDE_DIRECTORIES "${sodium_INCLUDE_DIR}"