Avoid overriding user linker flags (LDFLAGS) for modules and shared libs

Signed-off-by: Fabio Rossi <rossi.f@inwind.it>

CMakeLists.txt

@@ -286,7 +286,7 @@ add_definitions(
   -DHAVE_CONFIG_H
 )
 
-set( CMAKE_SHARED_LINKER_FLAGS "-Wl,--no-undefined" )
+set( CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,--no-undefined" )
 
 
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