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

Signed-off-by: Fabio Rossi <rossi.f@inwind.it>
(cherry picked from commit 84aad37849)

CMakeLists.txt

@@ -103,8 +103,8 @@ add_definitions(
 )
 
 set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${TQT_CXX_FLAGS}" )
-set( CMAKE_SHARED_LINKER_FLAGS "-Wl,--no-undefined" )
-set( CMAKE_MODULE_LINKER_FLAGS "-Wl,--no-undefined" )
+set( CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,--no-undefined" )
+set( CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} -Wl,--no-undefined" )
 
 if( WITH_TEST )
   enable_testing()