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

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

CMakeLists.txt

@@ -57,7 +57,7 @@ install( FILES ${CMAKE_CURRENT_BINARY_DIR}/dbus-tqt.pc DESTINATION ${PKGCONFIG_I
 ##### build setup ###############################
 
 set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -include tqt.h" )
-set( CMAKE_SHARED_LINKER_FLAGS "-Wl,--no-undefined" )
+set( CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,--no-undefined" )
 
 add_definitions(
   -DTQT_THREAD_SUPPORT