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

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

CMakeLists.txt

@@ -209,8 +209,8 @@ 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" )
 
 check_type_size( "long long" LONG_LONG )